There is a way to get round it, although they have upped the terms. Last time I went I parked in the big retail park right next door, where if you spent £20 in the shops there you got extended parking for free (2 hour limit on matchdays). Looks like the greedy buggers have now upped that to £50:
Match day and event parking
MATCH AND EVENT DAY CONDITIONS OF PARKING 2 HRS MAXIMUM STAY WITH NO RETURN UNLESS £50 OF PURCHASES FROM ARENA PARK RETAILERS.
Please retain receipts as proof of purchase for up to 14 DAYS. Unless you have shown your receipts to the Park Watch attendant on site and your registration has been noted by them or you have contacted Park Watch by email at info@parkwatch.co.uk or 01244 283 343.
Tesco have some good beer offers on at the moment, so stock up for Christmas!