It's a tad more than $3500 if you buy off the shelf and don't know how to mount everything, plumb it all up
Then you have to figure out how big you want your tanks, what size compressor(s) you want, duty cycle of compressors (running for long periods of time?)
You my also get frustrated with the speed of the 3/8 lines and valves and want to upgrade to 1/2". Factor in some cash for that too.
Then there is labour if you have no experience
Can be quite costly. At least it is just a straight swap over - not like other vehicles where new mounts have to be designed, leafs taken out, 4links etc.
Just some points to ponder..