Better lens and as cheaply as possible don’t go together. The cheapest possible lens is always the one you already have.
What you should not do is buy another lens because it is very marginally better on paper than your current lens. That’s how you waste money on GAS (Gear Acquisition Syndrome).
First, don’t buy another lens just because it is supposed to be a little bit better than the one you’ve got if it covers mostly the same focal length range as your 15-45mm lens. And nothing even remotely “cheap” is going to be much more than just a little bit better than the 15-45mm lens you already have. Not that there are any real choices within the Canon EF-M line since the EF-M 18-55mm f3.5-5.6 IS STM was discontinued not long after the EF-M 15-45mm f/3.5-5.6 IS STM replaced it as the most affordable “kit lens” for EOS M bodies.
There may be a few edge cases where a little extra aperture or extra rated IS may help, if such a lens were ever available in the EF-M mount or you decided to adapt an EF or EF-S lens to your EOS M50 body using the Canon EF to EF-M adapter. But those will be fairly limited to shots that will look barely better with a slightly better lens than the one you already have in very specific scenarios (say, handheld photos of static scenes in low light). Good technique can go a lot further than a minor difference in aperture or even one stop of IS does. If you’re using a tripod, as you should for static scenes in low light, or if your subject or other parts of the scene are in motion, then there’s no real difference between lenses with marginal differences in maximum aperture or IS.
The copy-to-copy variation from one copy of the EF-M 15-45mm f/3.5-6.3 to another copy of the EF-M 15-45mm, and from one copy of an EF-M 18-55mm f/3.5-5.6 IS STM to another copy of an 18-55mm STM can be larger than the measured differences between the EF-M 15-45mm and EF-M 18-55mm STM shown at sites like DxO Mark. So you might wind up with an EF-M 18-55mm STM lens that looks better at DxO, but the copy of the EF-M 18-55mm you actually get isn’t as good as the copy of the EF-M 15-45mm lens you already have!
This can be particularly the case when buying lower priced lenses used. Lots of folks will buy/return/sell lower cost lenses until they find a “good” one, which often actually means it matches up to their particular camera body and its variations within manufacturing tolerances better than another copy of the same lens does. But sometimes it means cheaper lenses that are slightly out of alignment show up more often on the used market than more pristine copies. Even with very expensive lenses, finding someone who can do an expert job of lining up a lens is difficult. With cheap lenses, it often would cost more in labor to properly adjust the alignment than the lens is worth, and that’s assuming the lens even has provisions for optical adjustments. Some lower cost designs do not. So folks will buy/sell multiple copies until they get one they want to keep.
Second, don’t buy a lens that covers the same focal lengths you already own plus more on one end. In other words, don’t replace your EF-M 15-45mm lens with an EF-M 18-150mm lens. The quality you get from the latter will likely be worse in the 18-45mm range they share in common than the lens you already have. Instead, add the EF-M 55-200mm. You’ll almost always have better optical performance from two zoom lenses that cover about 3X focal length range each than a single lens that covers a 10X focal length range.
If you really want to replace or supplement the 15-45mm lens with another lens that opens up possibilities the 15-45mm lens doesn’t allow, you should think more in terms of true qualitative improvements such as:
- Faster aperture zooms, like an adapted 17-50/55mm constant aperture f/2.8 lens. But keep in mind that typically landscape photography is done at narrower apertures, even when using lenses with wide maximum apertures. The differences in “sharpness” between cheap and expensive lenses usually disappear by the time they are all stopped down to f/8 or so.
- Wider focal lengths, like an EF-M 11-22mm or an adapted EF-S 10-18mm.
- Longer focal lengths, like an EF-M 55-200mm or an adapted EF/EF-S telephoto zoom.
- Prime lenses – that is, lenses that don’t zoom but only have one focal length – which often give better performance at a much lower price than high end zoom lenses do.
But don’t go buying such lenses just because someone else, like me, tells you to. Buy whichever one you need when you realize which one will let you do something you want to do with your camera that your current lens does not allow.
As we said a while back in our answer to this question about marginal upgrades in gear for a beginner:
Lens decisions are an intensely personal thing. What one photographer may consider essential can be totally superfluous for another photographer. The more you know about how you want to shoot, the better informed you’ll be to decide which lenses are best for you when the time comes to start spending more on gear. What one must be careful to avoid with this strategy is the constant desire to frequently upgrade to a slightly better lens (or camera) than what one is currently using.
and (slightly paraphrased):
At this point you don’t even know how much you’ll enjoy (or not enjoy) photography as a hobby. Assuming you do decide to stay in it for a while, you might surprise yourself with what kinds of things you find you enjoy shooting the most and what type of things you quickly grow tired of shooting. It would be a shame to find any lenses you wasted money on near the beginning of your photographic journey aren’t well suited to what you eventually find you most want to shoot.
In all likelihood, if you decide to stick with photography as a serious hobby, you’re going to outgrow either one of these lenses relatively early in your photographic development. Don’t waste money buying a redundant lens that is, for all practical purposes, no better and less useful than the one you already have!
In other words, start out at the ground floor and wait there until you know enough to know where in the building you want to end up, then use the elevator to go directly there instead of climbing the stairs one floor at a time using all of your energy (money) wandering around looking for where you want to go.