Usual active adulterants in street MDMA are: caffeine, BZP, TMFPP, amphetamine or methamphetamine. This is what I've come across in some of my personal experience testing pills with lab equipment.
Non-actives are also common: things like baking powder, sugar, leftover inorganic salts from synthesis (like aluminum or boron oxides) also random crap in small amounts that usually comes from inadequate purification during synthesis.
The super cheap pills will probably be BZP or a related piperazine. They are way way cheaper to synthesize than MDMA. Take a look at the wikipedia links in my above post, or search BL for BZP if you want more info on those.