This is the catalog of a Honda dealer that sells parts online.
Honda Automotive Parts

A complete set of mounts would be parts 5, 6, 8, and 9, which adds up to about $450 at the discounted price. (This is for an EX-- the Si is likely different I didn't look.)

Using genuine dealer parts, $600 for the whole job is not out of line. It would cost twice that at the dealer.

 

i would have charged a 30 pack of beer labor cost for a cousin ???

 

If you are still curious, look at your receipt (hopefully itemized). Look at the cost of the replacement mounts and call a dealership to see if the amount is same/similar. You could also ask service desk at dealership (or other trusted mechanic familiar with Civic) to get estimate of number of hours to replace all 4 mounts. Then see if the hours match up to your receipt.