It is not just a matter of putting a row of cringles in. The luff and leech need several layers of re-enforcement sewn on, similar to the existing clew and tack of your sail. And the reef points need patches of cloth to re-enforce them too. Given the time spent to do the cutting and sewing £75 seems quite reasonable. You could add just cringles, a sail maker usually charges £3 to £4 each, including riveting them, but I suspect any reputable sail maker will be unwilling to help you, unless you allow him to do a proper job.
Or as only the English language can express is: Don’t be penny wise and pound foolish.