You absolutely can shoot the ILS at an airport, "break-out" below the bases, request a special VFR from XXX-ILS airport to North/East/South/West to clear the class-D, then proceed to your destination VFR(in class-G, make sure there is no Class-E connected, could bite you in some cases).

Now, this situation is perfectly legal according to the FAR's, but you also need to consider are you:

A: Over a congested area, you now need to be 1000' above that, 500' over other area's is obviously the prefered route.

B: How well do you really know the terrain and obstacles? Before I'd ever attempt anything of this nature, I'd highly advise that you simulate this in VFR conditions, step by step, the planned route of flight, low altitude etc.. You need to see it in the good weather to really appreciate what's going to be coming at you in less than ideal conditions.

C: What's it really worth? If the ILS airport isn't really that far away, just land there, chill til the weather clears, or just plan on leaving the plane and getting it when the Wx improves. As a professional pilot, I've performed a few of these, and on several of these occassions I had to ask myself what the heck I was doing. Some things just aren't worth it

This was a very good question and topic, thanks for sharing and asking! If I could afford the plane, I'd say go with the IFR GPS, it'll come in handy in your regular flying anyways. Whomever told you $30,000 for a GPS installation is full of it. I know for a fact that at my company it doesn't cost that much. Now, if you want Dual WAAS GPS's, then it might, but I don't know what it is off the top of my head.