Best to order the board by the actual part number on the board sometimes you can just Google the board # and get even more results
Shopjinny.com is a well known parts re seller lots of shops use them they warranty parts.
Thermal sensor can fail either way hot or cold fail.
very simple repair to R&R power board .
customary mark up on used parts maybe 20-25% labor including diag maybe $100.00
Remember shop has to pay fixed costs + expenses just to keep the doors open to be there for you.
used boards are approx $95.00 new boards $200.00 +
new board http://www.partstore.com/Part/Samsung/Samsung/LN40A550/Samsung/Samsung/BN9401723K/New.aspx
Again boards should be ordered using the service part # on the board itself.
as to whether it is worth fixing hard to say for a hundred or a little more maybe
maybe not if it is just going to fail again you can get a 39 inch Toshiba LED
on sale sometimes for $299.00 , a 40 inch for a little more .
maybe ask repair shop if that is a recurring fail on those sets.
or maybe Google will know.
Toshiba at least for me have had good pictures and very reliable.