A friend of mine had the mainsail on his westerly nomad badly ripped. He took it for repair to a "one man band" sailmaker last week and he made a fantastic job and charged him £60. Seemed like a good deal to me.
It depends on the size of your sail of course (mine is 145 sq ft according to sail plan but a roller furling genoa may need to be shorter in luff to accommodate drum, swivel etc).
I have brand new Kemp hanked genoa cost £390 to build.
Kemp Quote for modifying as follows:
"Luff re-cut @ £85.00, ex vat.
New UV protective strip at foot and leech @ £110.00, ex vat.
If you are using an existing reefing gear we need to know the following:
1) max luff length
2) rope diameter
3) direction of rotation of drum"
You might well find independent sailmaker with lower overheads (and without the incentive of trying to make you buy a new sail instead of modifying and old one) cheaper.
The other thing to consider is the resale value of your old sail(s) but unless you know someone who wants a hanked sail it is likely to be difficult to shift them. Everyone shifting to roller furling wants to dispose of old sails.
In my experience secondhand - even roller furling sails -
can be hard to sell - I've still got a main and furling genoa from my old Westerly Konsort which I will throw away if nobody makes me an offer soon.