Ok here is a simple way of costing your restring,
Polyester will stay in it's tension range for 10 -14 playing hours and costs £16 therefore the cost per hour is £1.20
Peek (Ashaway Monogut ZX ) will stay in it's tension range for 30+ playing hours and costs £24 cost per hour is 80 pence
Nylon based strings will stay in it's tension range for 25 playing hours and costs £16 cost per hour is 64 pence
Natural Gut will stay in it's tension range for 35 playing hours and costs £32 cost per hour is 91 pence
There are other strings that will stay in the tension range for 25+ hours: Luxilon Alu Power Rough, Babolat Origin etc
This I believe is a better way to cost out your restring.
Please note that playing hours is also dependent on your playing style and your rackets string pattern.
(example 18 x 20 string pattern strings should last longer than the 16 x 19 string pattern)
Hope this helps!