So far, I'd guess I am at the longer end of the budget.
Hotel at start/finish - £350
Hotel along the way (Loudeac * 2 nights) - £100
I'll sort out travel some time soon - decide how many days to take getting there and back, that will sort of decide how much I might spend on that - but certainly another £300 with ferries and hotels.
On the ride - probably spent around £100-150 last time? I really can't remember. Food in controls wasn't cheap, but wasn't insane (and was fast and generally pretty good) - I think I would typically spend 10 to 15 Euros in a control where I wanted hot food, pudding, drinks and a snack for the road. Depending on the kind of ride I go for this time, I'd like to sample more restaurants, patisseries and cafes this time.
It's not a cheap bike ride, though camping etc. your way down there and back etc. will minimise the costs. I think you'd struggle to do it on much less than Frank's amount above (and that probably doesn't include the £100 or whatever of entry fee?). Of course, the strength of Sterling/weakness of the Euro may make it somewhat easier this year.