How much will it cost to fix my iphone 4? - 1

I was out last week and it noticed that my phone had stayed on 33 percent charge for ages even though i was using it alot.
later on my phone turned off, and when i plugged it into charge would show apple symbol and then go blank and kept repeating this process.
i decided to restore my phone to factory settings to see if this would help
my phone would turn on (whilst on charge) but then turn off after a few minutes. Every time it would turn on, the percentage of battery would be significantly different. For example, once it was 100, then 1 then 7, then 14.
Now, my iphone wont turn on, and only shows the apple logo for 3 seconds when i plug onto charge an then goes blank.

if you can work out what is wrong how much would it cost to fix this issue ( in a market stall or small shop)

also, the lock button is broken how much would it cost to fix that too?

my phone is pay as you go and the warranty has ran out.

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