I guess it can depend on alot of factors and often some dealers do deals on the advance payment. on my last car i got £200 off and the one before that £250 off the AP. I never add any extra’s, i chosse the cars that have what i need and want as standard. You can use this link to see https://www.motaclarity.co.uk/news/motability-deals what deals are on offer and where they are and some users have travelled to get a car, cheaper and i guess if you save on the ap you can use that to offset any extra’s but i just see them as added money you never will get back after the 3 year lease and mb will benefit from.