I have an HP printer and a subscription with them whereby I pay a fixed amount per month to print up to a certain amount, and they will automatically send a new cartridge when my machine tells them I'm running low, and even include an addressed envelope for me to return my empties for recycling. This works very well for us, as we only do occasional printing for personal stuff, and my subscription is just £1.99 a month, and there's no walloping cost to pay out for new cartridges when they need changing. Of course, if you are printing loads for work, a bigger subscription may not work out as cost effective.
Yes - we have this. I was initially quite sceptical - but then I discovered that they basically just charge you £x for up to y pages whether they be draft or best quality, B&W or colour. Which has revolutionised my photography as now I can print 50 A4 full colour prints on photo paper for next to nothing (same price as 50 B&W draft pages of type for instance).