Doesn't its value depend on how many Avios you're going to get rather than the mere fact that you have to pay it? After all, you can pay with a debit card at no extra charge.

If the transaction's only going to earn you about 200 Avios (as it might before DVLA accepted Amex), then £2.50 doesn't seem to be particularly good value. On the other hand, if you have a bigger amount to pay and you can now use Amex and earn (say) 1000 Avios, then the £2.50 fee would seem much better value.