Under 15 bucks,
Well, if you have a local B&M shop nearby hit them up, sometimes ( more often then not) the folks hanging about will have excess hardware and they may have something for you (sometimes its really nice for rather cheap).
See if you can find an AGA-T, its a long shot but by now they should be around 15 bucks.
Though RBA covers 2 branches in truth...
RDA-Rebuild-able Dripping atomizer, these can be sourced for under 15 bucks easily, they don't have a tank but will do the job just fine without too many hassles, my preferred go to actually.
RTA- Rebuild-able Tank Atomizer, these have a tank, either top like the kayfun orchid and Russian 91% or bottom like the RSST, Steam Turbine and AGA-T. the differences are pretty major though, most bottom feeder tanks will use stainless steel wick or the likes, I've heard tale you can use these with cotton but again, never tried it myself.
Top side tanks use cotton, Rayon or Silica to wick the liquid to the coils, they usually have a channel that feeds to juice to a well inside a Chimney where the wicks will pick it up and transport it.
Sorry if this is information overload, first would to determine what type of RBA would fit your vaping style then can progress from there.
Hope this helps,
Sincerely,
Midnight