A 36 or 55w retrofit kit will cost you $42.
You'll have to check to see which will fit the dimensions of your tank.
Bulb will also be extra.
You have not explained what type of CO2 system you intend to spend $35 on.
DIY would be an area to economize here, but you said you don't want to.
You will need to spend some $ on ferts. Needs will differ depending on your plants. Dosing macros with bulk chemicals is definite way to economize. With bulk macros and additional bottles of micros, iron, etc., $10 per month would be a very high estimate.
Your plants will cost something. You can spend just about as little or as much as you want. Per plant cost can vary from $.99 to $10. You can start off with very few plants and spread them via clippings as they grow, or you can start off heavily planted from the get-go. For a 20g, I'd estimate $30 could get you started okay. I recommend starting on the slow side, then increasing number and variety, as it is easier to troubleshoot. OTOH, heavy planting can guard against algae.
You'll also want to consider test kits. People differ on exactly what is required. Bare minimum IMO is NO3 and ph. Say $12.
That's about all I can think of.