Currently shell v power where I live is 19p per litre more expensive than Tesco normal petrol. So a full tank would cost £15.20 more to fill up with v power.
Notsure what the cost of Tesco 99 is, my wife fills up her 206 with Tesco normal and I fill up the Qashqai with v power. Doesn't need it but I truly believe in the benefits it brings. Treat your car right and it'll last longer.
Looking back over my old fuel economy figures on the zed and on the assumption of doing 10,000 miles a year. It will cost you £434 extra to fill up with v power over Tesco normal petrol. It's a 30k car, so needs a bit of extra love IMO. That and £434 over a year for a car doing 10,000 miles is not too bad.