I know the basic laws of supply and demand tell me that trailers will be different prices everywhere, but let's set that aside for a minute.  I'm looking for a used 12 foot utility trailer to pull my jon boat.  About how much should I be expecting to pay.

I live in Michigan if it makes a difference.

Around here that might be a pricey item.  There just are not that many 12' trailers used.  Here 8', or 16'+ are the most popular so the 12' maybe either a homebuilt or specialty class trailer.  Urban, lawn care and 4 wheelers use most of the smaller ones and farm and rual use the larger.  One could be built fairly reasonable.  There are "alot" of places here that build trailers, but used ones can be hard to find at times.  I know they sell kits and you could use lighter materials (if you have the equipment).  I would think you could find a boat trailer alot cheaper if that is it's only purpose.  I am just guessing $100.00 -150.00 for an older used boat trailer.  Maybe around $250.00 - $350.00up for a used utility in decent shape.  Like I say those are just guesses.

Not sure if you can down size or consider cheap but Harbor Freight has some pretty good deals.

http://order.harborfreight.com/EasyAsk/harborfreight/results.jsp

Check NADA guide for used prices....The prices are specific to your area.

http://bassresource.com/fish/boatprices.html   ;D