£20 for the 10gallon bucket, £8 for twin elements. I don't use a thermostat and don't really see the need for them unless in an old school electrim mashing and boiling bin which I don't rate anyway. I get a good strong rolling boil off one element but use two to bring it up to boil faster before switching one off.
Some people use those big blue 10 gallon buckets which can be had off eBay for about £5 which brings the cost down even more. You would need to fit a tap and ball valves are the most commonly chosen for reliability and ease of fit and you can get those at a plumber's merchants for about £5.
So if you went down the 10gallon white FV route with twin elements, re-using your hop strainer and fitting a ball valve it'll cost about £35.
A blue bucket off eBay about £10 less than that to account for delivery.
What's your mashtun size currently? Do you batch or fly sparge? I'm batch spargin at the moment as my mashtun has a 10gallon capacity and I can't really be bothered to rig up a fly setup but you can get greater grain into your mashtun if you use fly sparging as you don't need the room for the sparging water.