Like most things, costs are relative to the desired resultant.
 
A typical 100% electric vehicle conversion for the D.I.Y. is typically $15-$20,000 with a range of $500 to $50,000. The advantage of a 100% conversion is obviously a pure electric vehicle. The disadvantages to a full conversion are of course range and performance unless price is not a factor.

Converting a vehicle to a hybrid can be considerable less expensive without sacrificing range or performance. To the contrary, a hybrid conversion will only add to the range and performance of the existing I.C.E. (while additionally allowing the vehicle to be somewhat rebalanced).

Costs for a hybrid are again dependant on the goals. However a smaller electric motor, a smaller pack, and a smaller controller will easily save 50% to 70% on the electric components for a hybrid.