Help i have just been dumped wiv a plecko?

i have just been dumped wiv a 14 inc long and 4 inc wide plecko but its far to big for my 3 foot fish tank i was wondering how much would u get for it from a aquatic place when i take him in tomorrow in a bucket i have had to put him in my tank for the time been cos he was left on my door step in a bucket does any one have any ideas or does any one know how old it will be cos it had a note left wiv it sayin hes a sucker fish grown to big for me tank found out u had a fish tank so here u go