Bottled LPG gas

This really depends on where you live - It's no good me saying that a place near me sells bottled gas cheap if you live miles away!

Have you got an empty bottle to exchange?
If not you can save yourself some money first by trying your local council tip/recycle centre to pick one up.They'll charge you a lot less than the deposit.

13kg is a propane size that’ll likely have a screw fitting. A similar size butane bottle will have a clip on regulator.
Calor tend to be a bit dearer (around £20) but more readily available (again depending on where you live).
Some of the other bottled gas companies are often a bit cheaper (what brand do your local garage & hardware store supply) but here again, they’ll want a deposit for “their” bottle.
Local paper & caravan sites can often be a good source. Also try a “local” search on eBay you may pick up a real bargain because of the local collection only bit.

I pay £18 for a 19kg propane, the advantage of this size is there is no deposit to pay either, they take a different regulator though (the screw in type you fasten with a spanner, we use it for our BBQ, I prefer this type anyway as it is more secure) but at £5.50 and a one off cost they are worth it, have a look here http://www.flogas.co.uk/ see if they have any local to you.

Propane also work more efficiently at lower temperatures as they dont lose the pressure the same as butane does when it gets cold.

Dont be too sure, Ive seen empty bottles go for as much and more than the dealers charge for depositsthinking:

+1 the larger sizes are much better value but OP never said what it's for, just asked about 13kg size, so if it's got to fit "inside" something, 19kg may be no good.

Well... obviously, use a bit of savey don't go bidding more than it's worth... But at the same time I've seen a full bottle go for a 99p start price - I wanted to bid, but didn't live anywhere near.