First—as noted previously, inflatable PFDs don't make any sense for dinghy sailors... dinghy sailors get too wet for even the hydrostatic release ones to really make sense.... the refill kits range in price from about $19 for the older automatic salt-pill type inflators, to about $50 for the automatic hydrostatic release inflators.
If you do get one for use on a bigger boat...get one with an intergral harness... they're much less of a PITA than a separate harness and PFD...and most of the time, when you'll be wanting a PFD, you'd also be wanting a harness.
Also, reduces the number of things you have to carry or wear.
I would second/third there being little if any point to getting a manual inflatable... too many variables could prevent manual inflation... automatic is much better IMHO.
If you do get an inflatable, I'd also recommend trying it out in operation... jump into the water in a protected anchorage and see how it works... it is quite an experience...and better that you have an idea what will happen before you need it. Buy at least two refill kits and keep them aboard—preferably in waterproof containers. I highly recommend vacuum sealing them—as it prevents corrosion and the humidity from affecting/degrading the salt tablet in the older variety of automatic triggers.
BTW, the cartridges, tablets and other parts to the recharge kit are fairly specific to a brand or model... so get the right one... and check the fit.
Finally, if you do get one and use it.... check to make sure the cartridge is fully engaged/screwed in... in many cases, I've seen them where the cartridge is only partially engaged and if the vest triggered, it would either not inflate or lose much of the gas and only inflate partially. Get in the habit of doing this every time you put on the vest.... it could save your life.
BTW, most airlines have a specific policy on carrying the CO2 cartridges aboard. You have to check with each airline. FAA regs say that you're allowed to carry two cartridges on a given flight, but some may require you check the cartridges as baggage, others require that they be carried on.