What are you hoping to accomplish with another lens? What are you not able to do with your 18-135mm? After all, you already have 50mm and 24mm focal lengths with the zoom.
If you want a large aperture lens to shoot in low light and/or blur down backgrounds in candid portraits, the EF 50mm f/1.8 STM is a pretty good value. 50mm is a "short telephoto" on an APS-C camera like the 80D. Personally I prefer the EF 50mm f/1.4, but it's about twice the price.
Not sure about the EF-S 24mm f/2.8 STM.... it's an extremely compact "pancake" lens, if that's what you have in mind. However, you already have 24mm f/3.5 or f/4 with your 18-135mm. Only 2/3 to one stop difference. Not a very big deal.
If yours is the EF-S 18-135mm IS USM, it's probably faster focusing than either of the STM lenses. Neither of those primes have IS either, while you're zoom does.
If a 70-200mm is too rich for your budget... how about an EF 70-300mm f/4-5.6 instead? The latest version "II" is currently on sale. The previous version is still available new, and a decent lens too... for under $400. Both 70-300s have fast USM focus and helpful IS. Once you save up a bit more, if you still want a 70-200L (I recommend the IS versions with either f/4 or f/2.8), you can sell off the 70-300mm.
Another response suggested the EF-S 55-250mm IS STM... which also might fulfill your telephoto needs quite well. It's a very capable lens, too. About $300 for one of those.
Of course, there quite a bit of overlap with your 18-135mm and either 55-250 or 70-300mm.
An alternative would be the EF-S 10-18mm IS STM.... an ultrawide that will give you a range you haven't currently got covered. That would be nice for scenic shots, among other things.
Another option would be a macro lens, if you like to do close-ups. The Canon EF-S 60mm f/2.8 USM is quite compact and might serve some of the same non-macro purposes as a 50mm lens. It's on sale for under $400 right now. (I opted for the Tamron SP 60mm Macro instead, because it has a larger f/2 aperture that makes it even better for dual purposes such as portraiture... but it's more expensive than the Canon lens. It's slower focusing than the Canon, but that's no problem for most macro or portraiture.)
Or maybe you just have G.A.S. ("gear acquisition syndrome", highly contagious and common here on UHH
). In that case, do you have the matched lens hood for your 18-135mm? If you don't, I'd highly recommend getting and using it regularly (same with all the above lenses... except possibly the EF 50mm f/1.8 STM, which has a fairly well recessed front element, so may not need a lens hood). Or, do you have a high quality circular polarizer for use on it? With digital photography, a good CPL (such as B+W F-Pro or XS-Pro) is the most useful of all filters.
Just tryin' to help you spend your money!
What are you hoping to accomplish with another len... (