I have not seen reports on the costs of the Pirelli's, (they will present to the teams @ the Spanish GP), but the Avon tires are reported to be 1/3rd the price of the Michelins
As Sam Michael alluded to in that linked article, the open question is what will be the quality of Avon tires?
Tire quality problems happen to the best of manufacturers. An obvious example would be the failing Michelin tires at Indy in 2005, though there are plenty of other examples of course (I don't mean to pick only on Michelin here).
Avon tires are used in various racing series, so they know racing. Their pricing presents a pretty strong incentive to go with them, but they ought to provide evidence to the teams and FIA of a rigorous quality program that ensures a high level of parity, and strong engineering abilities to ensure that they do not create their own Michelin Indy 2005 moment.
As a fan, the brand matters little to me, as I'm more interested in parity
(all sets of tires of the same compound should be very, very similiar). And second, I'm interested in safety
. The obvious worst case safety failure would be showing up to a venue for a grand prix weekend, and all tires on hand are not structural sufficient for the venue (aka the Indy 2005 scenario). But beyond that, safety concerns would include tires which deflate for other quality reasons, and cause a high speed crash and injuries or death.