1) Any aquatic plants are betta safe!
2) Not sure on cheaper plants, depends on retailer, but every plant has a "keeping" level of either easy, medium, or hard. Some easier plants are Jungle Val, Java Moss, Java Fern, Anubius, Water Sprite, etc. If you look around on Google you will find tons of plants.
3) Should be fine for most easy-keeping rooted plants
4) Yes, you will need to dose you're plants either once or twice a week either with liquid fertilizers or root-tabs. I use a fertilizer called "Seachem Flourish" which is a pretty standard fert. that alot of people use. Where I live in Canada, its about $7.99 give or take for a medium size bottle which lasts me for the better part of a year, so it is definitely affordable.
5) It all depends. Again, plants have a low, medium, or high level of light. I would consider asking the people that work at the retailer what light level the plants you're interested in need, or else they will rot and leave a mess in you're tank. What kind of light setup are you using now and how big of tank?
6) Mystery Snail should only really eat the decaying plant life but might eat the leaves of plants, depending on how much food is already available.
7) it will most definitely make you're betta ALOT happier!
8) It will only really make your water for oxygenated and better for fish. Also might make your water more crystal looking.
9) Depends on what your water change schedule is already and how many plants you are keeping
What part of Canada are you in? Also, plants can be quite affordable if done right. Also, if everything is going good in your tank and your plants are doing good, they will send out runners (Or split) and produce new plants. I bought a Jungle Val for $2.99 and now I have 8 of them!
Canadian Fish Owner