Is the phone replaceable? (Broken lock button, no warranty)

Hi, I bought an iPhone 4S off a friend about 4 months ago, (the phone already had a jammed lock button) and it has been great until yesterday when it suddenly turned off itself. ( I can't turn it on manually because the button is not working) so I plugged it into the charger and it started to turn on, then turned straight off again. It kept on doing this all night. It has no warranty and I was wondering if it would be replaceable ? If so how much will it cost? Thank you your answers would really be appreciated.