Probably the cheapest you can do nowadays is getting a pack out of a wrecked Leaf or Chevy Volt. The downside is the automakers tend to use LiCo batteries, which are more prone to fire and have shorter cycle life, but do have good energy density. You should use the original car's monitoring or have a sophisticated monitoring system.
If you are a pure racer, for sheer power-to-weight for a good price, it is tough to be RC LiPo batteries. They do not have good cycle life (OK for the RC crowd), and are easily damaged with their soft envelopes.
LiFePO4 (lithium iron phosphate batteries) are good hobbiest batteries. They are more resistant to fire, have great cycle life, and are pretty inexpensive. They are also what most hobbiests use (Charles being the latest and greatest local example, hopefully me soon), For power $/kW, Headway Power cells are cheap, but you have to string many small cells together. If you want range $/mile, the big block cells like CALBs that Jack Rickard sells or HiPowers (not really high power but are more for range) cells that Carl sells are inexpensive for new cells.
If you don't kill them, LiFePO4 cells are cheaper in the long run than lead acid!