Pentium 4 was available in Socket 427, 478 and LGA775 packages. 427 parts only went up to 2.0GHz, so your current processor can't possibly be that. Socket 478 and LGA775 have great deal of overlap in available part specs. One way to tell is to remove the chip. If it has pins, then it is Socket 478. If the chip only has pads on the bottom, then it is LGA775.

Froogle shows some vendors sells refurbed 3GHz Pentium 4 at around $30, such as Geeks.com. $120 sounds way too much for old technology.