i think your refering to prochems; the top end is 50sqm per 5 ltr tub,
you should be able to get a bit of discount from your regular chems supplier, say 10% off the £29.50 per tub, you should find a good enough profit charging £1.50 per sqm protection.
chems = £106
charge = £290
or for that large area maybe you could charge less, say £1.10 per sqm. which will be a bit more acceptable for the customer.
chem = £106
charge = £214
regards,
craig