The included spreadsheet below is divided into two columns to support both arguments - the column on the left is the traditional view that the cost of the whole kit must be fully recuperated by the reduction in gas prices. The right column considers that the kit will always be worth "something" on the second hand market when you come to sell the van and in my experience you can expect to recoup at least 70% of that cost especially if Gaslow continue to increase the price of their kits.
If you want to have a go at this yourself you can Download the Excel Sheet (Right Click, Save As) or alternatively if you don't have Excel Installed Edit the Public Version on Google Docs.
Note: You never actually get your full Calor bottle deposit back so for the purpose of this calculator I will ignore this for two reasons. 1.) If you are just starting out, you are probably going to be motorhoming for several years and therefore after a term the Calor bottles will be worth next to nothing. 2.) If you already have bottles you need your original paperwork in order to get a refund therefore that money is "locked into" your bottles.
However if you want to factor it in and you have, or will have, your original paperwork see this table from Calor: http://tiny.cc/CalorRefund. If you don't have the paperwork, Calor will donate £5 to charity.
Can you put a price on convenience? Does everything you buy always have to save you / earn you money - can't you just enjoy the benefits it brings? These are some of the arguments I've seen. If you can't justify it on repayment alone then you'll have to put a price on the convenience and decide if you feel it is worthwhile.
Price of Gaslow Kits: http://tiny.cc/GaslowKits
Price of LPG Gas in Europe: http://tiny.cc/LPGPrices
Find your cheapest local LPG supplier: http://tiny.cc/LocalLPG
Calor Refill Prices (use RRP): http://tiny.cc/CalorRefill
Calor Deposit Refund Agreement: http://tiny.cc/CalorRefund
Excel Calculator Download: Right Click, Save As