Like Calor it tends to be a reseller can choose their own selling price so the price asked varies.
I use the 10 kg size and changed two, for an agreed £38 a piece, a few months back.
I recall B&M were asking £43, and the Flo gas depot were asking even more.
What is for sure is that over the last two years the price has rocketed, as for years I exchanged at a steady £28.
Another aspect that grinds with me, is the more commercial Flogas steel 11 kgs propane, so actually getting more product, can be had from £30.50 to £34; there is clearly at present a full-on exploitation of leisure users in the industry