Firstly, I very much doubt AV will cost the £250m the No2AV camp claim. I explain why on my campaign blog here.
Even if we decide to introduce electronic counting at the same time as AV, that's 2 distinct decisions with 2 separate benefits. I've observed a general election count - I can see benefits of electronic counting even if we keep the existing first past the post voting system.
Besides, council elections are happening in many parts of the England, plus there's already Assembly elections in Wales and Northern Ireland and Scottish Parliament elections on May 5th. It's far cheaper to hold a referendum at the same time as other elections, 5th May 2011 seems an ideal date!