Sticking to round numbers, a rented 172 @ 100kt @ £200/hour makes the sums very easy - £2 per mile. (The ones I fly go slightly faster than that and cost slightly less than that, but only about enough to take account of the faffing around on the ground at each end of each flight.)
And if you're looking at 50 gallons capacity at 10gph and a one hour reserve then in round numbers you're limited to legs of around 400 miles anyway. (You can do better than this, perhaps quite a bit better, if you're good at leaning and know your particular aircraft's performance, and don't pick a day with a headwind.)
Then there's whatever the owner charges for having the aircraft away from base. If there's a minimum charge of three hours per day (about what they could get if the aircraft remained at the club) regardless of how little you use it then your "5 nights at each field" could get very expensive indeed.
You might find a better deal than that - I know one 172 that's available for £99/hour plus £99/day, but I'm not sure that deal would extend to taking it out of club use for the best part of a month!