Check your local building codes first - not allowed to put a trailer on a permanent foundation in certain zoning areas.
Depends a lot on your local prices - and of course assuming you do not hit bedrock, but can run from about $15-30/SF in many areas to build a full basement, up to $40-50 in high-priced urban areas (which you likely are not in), plus probably about $1-2,000 to actually set the trailer in place on it.
Be sure the people who are going to set the trailer (may be contractor, may be a trailer moving/installation company) coordinate on the utility connections (so they are moved if needed so as to not interfere with foundation), and on the tiedown bolts/anchors so they line up right.