One member (notanewbie) here had a permanent bulkhead in his sprinter. He had a regular window a/c installed in the bulkhead. A small window unit would be enough for your small space and it could be ran with a 2000 watt honda generator. The a/c would be cheap ($200-$300?), the genrator costs $1,000 or less (i think) and it could be stored inside if you didn't want to carry it on the front or rear bumper.
If you want to carry it on the front bumper, you need a cargo carrier built for the front. There are a few threads here about the different setups. Most use a front mount receiver hitch, then just mount the carrier to that.
So, for a round number.... I would guess $1,500 - $2,000 turnkey.