the going rate is about $100-120 or so for shot peening 4 rods. I'm sure there are other cheaper places, but thats the avg rate.
on top of that, to strengten the ls rods, its recommended to upgrade to ARP rod bolts.
thats about $35-40.00

To use the ARP bolts, you cant just screw them in, you have to machine them to fit properly. If you dont, it distorts the rod bearing area and it goes out of round. This service is another $80 or so.
So that puts you about $250 just to shot peen and strenghten the LS rods.

If you arent planning on making over 250hp, the STOCK LS rods are fine. If you are boosting or making plenty of HP, then you should be going with aftermarket rods like EAGLE rods already.

So to sum it up:

Either you stay with LS rods or go Aftermarket.
Its not worth it to shot peen and strengten the stock LS rods.