Ok, lets forget about used accordions for a minute. Its true that you can find a good one now and then but it implies a certain knowledge...and a lot of luck. And generally, repairs are very expensive. I know you are correct but I was talking about brand new, very basic accordion prices.
The fundamental issue is that an accordion is a complex mechanical device with hundreds of moving parts, unlike, say, a guitar, which is essentially a wooden box and six strings. You can buy a new £50 guitar. You'll never be able to buy a new £50 accordion.