Yeah I assume that ME3 is no longer expensive but my PC is 5 years old iirc and of average quality, so I'm not sure it could run the game in very good conditions. There is also the "transition cost", having never played on PC I'm afraid I would pass from a guy who can solo platinum on consoles to a guy who need a full team on bronze on PC.
Maybe I will be tempted by the PC experience when I will buy a new computer, I don't know...
Pricewise, ME3 was on sale on Origin recently for 5€, and it happens regularly.
Transition wise, yes, it takes a little while but you do learn to switch and soon you'll be dodging and rage gelling without having to think about the keyboard.
Running wise, if you turn off anti-aliasing and dynamic shading, run fullscreen and tweak the resolution down if necessary, and use a booster like Razer Cortex, nearly any PC from the past 5 years will run the MP. Someone here a while back said they even had it running on a laptop with Intel GMA graphics.
Remember also that, once you have the game registered on Origin, you can install it on as many PCs as you want, so you can try it out now and then put it on any new PC you buy/build without having to shell out for another copy.