true cheapest place would be your garage if you can do this stuff by yourself, but if you can't then there really isn't a cheap place. I'd go buy some fel-pro head gaskets (around $17 each) and then bring them to a shop. If you know someone that works at a shop then maybe you can get a deal through them, otherwise it just depends on different shops for the amount of $$$
ive replaced probably 20 sets of v6 mustang head gaskets. did coolant get into your oil? does the oil look like chocolate milk? or the coolant? if they do, then you for sure did whats called a bearing wash. coolant got onto your main and rod bearings, and a layer of the tri metal bearing is now rubbed off. if it has, putting head gaskets in it is a band-aid, you will need at least a short block. i dont know your experience with cars, but if your not totally familiar with what im referring to, have it checked by a technician. better safe than sorry.\
if you only need head gaskets, i know some ford techs that will do it on the side for cash. i would love to do it, but since im turning a wrench for ford anymore, i dont have a lift. let me know what you find, and if i can help, ill be glad to.
take it to ford, make sure the odo reads under 100k by whatever means possible, I got the a new motor in my 94 v6 without any issues, I went to don davis, ford has a hush hush recall on it so it was totally free, what a deal
oh yea, the guy i talked to was jeff in service up at don davis, push for the motor, if your motor is knocking or anything it shouldnt be a prob but they will just try to only replaces the HG unless something else is wrong that they can blame on the HG...