My friend lives near Manchester. He’s a regular TT / MGP marshal who’s decided to marshal at the 2017 NW200 and he gave me details of the relative costs.
To take his motorhome over to Ireland and back will cost around £200 and, as he’s marshalling, he been allocated a place in the paddock so no camping costs. Total cost around £200.
To take his motorhome over to the Island and back twice (TT / MGP) will cost around £450 x 2 = £900. Camping costs for his motorhome £19 per night x 10 nights = £190 x 2 = £380. Total cost £1280 - £100 IOMSPC marshal’s discount = £1180.