You can find 14 of the 60 AH Leaf modules for about $80 each. A basic 14 cell BMS and the hardware/relays/connectors/cables can be bought for under $100. So it can be done for less if you shop right and don't mind doing the work yourself.
Once you factor in the efficiency losses of using lead/acid batteries in the RXV, a 60 A/H lithium pack is slightly better than the usable capacity of the average RXV pack.
Here is the start of a lithium conversion that I did to my 2008 RXV...
http://www.buggiesgonewild.com/elect...ml#post1505943