I went from Glasgow to Gateshead to get my Ioniq 6 cause the Evans Halshaw dealership was matching the £750 from motability.
With my 2 previous EVs I’ve seen deals on offer and phoned around to see if any nearby dealers would match it (or get close enough to make the travelling not worth it?) and both times they did saving £1200 on a Leaf & £800 on an MG5. This time they only offered £500 off but only on stock cars that weren’t colour or spec I wanted.
So to answer your question there is no set amount I’d need to save. It’d depend on whether the amount saved justified the time and expense incurred (I went down 3 times cause wanted to check in with the hand control fitrers) £50 probably not, £500 definitely 😁
Also to add I like driving and visiting places so none of my trips to Gateshead were a chore, I had a day out using various different routes that included visiting a dealership.