£75 might sound a lot but its not if you think about it. it is 236 miles each way from Folkestone which is roughly £100 in fuel for the round trip (assuming your motor does 40mpg & you pay £1.40 a litre) plus the tolls which are £30 from memory? Crossings vary between £41-£169 each way depending on when you book, due to holidays popular crossing times etc. plus you have to drive 4 1/2 hours each way...
To me it would be well worth saving the £60 (assuming you got the cheap time crossings) by going by train just to not have to drive back after the event... But hey if you cycled there I recon you might get there for less...