There's a Gas place just up the road from me and they charge About a ton for a 9 ltr that's it, can't remember refill price but you get your money back when you're finished with the bottle. I use very little gas at the moment, I'm with Energas and that's about £25 a month for 20 litre. Think I'll kick them into touch.
The hobbyweld ultra deposit is expensive but the gas works out cheaper.
It also includes a half decent regulator and a nice quick release fitting which helps justify it a little.
Also, if you have to pay delivery charges then a bottle with 3x as much gas will save you 2 delivery charges.
The rental/deposit on bottles is mad some places, seems to just be a licence to print money.
I get my welding gas from a place that supplies pubs and breweries and they charge the same as they charge the pubs for bottle rental.... £10 a year.
I ended up with Albee from energas/airliquide. Not the cheapest for the first bottle as it's a purchase rather than deposit but the gas is much cheaper. Regulator built into the valve.