Is the rate you've stated your normal incall rate? Or is that your outcall rate and then he's offering extra on top to cover the long travel time?
For me, my outcall rate is £50 extra for any distance of 30 mins travel time each way. For one hour travel time each way, I charge extra £100 and and up to 2 hours travel time each way is £150 extra. The way I view it is, if one outcall takes me 4 hours travel for a one hour booking, I could essentially take 3 hour bookings as incalls - and I find travelling just as much "work" as "work", so it all has to balance.
I would say just what you are happy with? If you are refusing other bookings to do this outcall, then is it financially worth it? If not, then accept the booking with the extra charge of what it will cost you in time and money to get there?