SPC is the abbreviation for Specialist. Currently, there is only one grade of Specialist - E4. Specialist grades used to go all the way up to E9 (although the Specialist 9 or SP9 was only theoretical - nobody had ever actually been awarded that rank). At that time, the E4 Specialist was abbreviated as SP4. However, all the Specialist grades above E4 have since been done away with, so there was no need to indicate the pay grade with the rank, thus the abbreviation became SPC.
In Pensacola, FL the mixing plant is currently charging $70 per ton for SP12.5 and about $67 per ton for SP9. Prices link with oil obviously. You can search for asphalt calculator to get a conversion from yd to ton. Materials are priced by weight.
Nonexistent. There used to be several grades of Specialist ranks. They would be paid the same as the NCO rank in the same pay grade, but would not be NCOs. They would be SP4 (E4), SP5 (E5), SP6 (E6), SP7 (E7), SP8 (E8), and SP9 (E9). Eventually, most of these were done away with, leaving only the SP4 rank, which was known simply as Specialist, and the abbreviation changed to SPC. There was never a Specialist 3 (SP3) - the only rank in that pay grade is Private First Class.