£410 is only £50 per door. (Including frame & materials)
That would be manageable if it was simply a case of putting the paint on.
Then there's: preparation/rubbing down/stopping in
That's assuming there's no change of colour involved (that can put the cost up a lot.)
Hopefully someone else will see this and give their opinion (ken4155 maybe?)
If you want a good job you have to pay for it. If this chap is willing to do it for 410 then I doubt he is a professional, certainly not of any repute anyway.
My council has a list of reliable, fair tradespeople and I have found it invaluable in the past, I cannot stand the thought of being ripped off.
It might be worth checking your council's website or giving them a call.
Age UK may also be helpful, I know they have a list of people who do smaller jobs for a reasonable fee.