Well, this question was asked a while before I joined and while I'm sure that your particular situation has been resolved by now (at least I hope so) I'll share my experience and solution.
I noticed my filler neck had rusted to the point where it was spilling out during fill ups. First, I used JB Weld to plug the cracks and seal the pipe, wrapping the works with Gorilla Tape for an added measure of protection (after clearing out all the crud which had collected over many years between the body & filler neck). After a trip to Alberta for work several months later, I noticed that fuel was again leaking while I filled up. I pulled the cover off and inspected as much as I could and found that rust had completely claimed the bend in the pipe and had spread nearly the full length. The breather tube was gone completely as well. I attempted another quick fix using more JB Weld but the pipe just disintegrated in my hands.
A trip to the local wrecker left me speechless - he had a few Previas in the yard and wanted $150 for the filler neck - and I had to pull it myself, if he pulled it he was asking over $200 and said he could get $185 for it all day long. Not having that kind of cash available, I decided to take another route.
After pulling the remains of the pipe out and inspecting what was left, I had the upper part where the nozzle goes in to the filler down to the section which shrinks in diameter and the breather tube splits off. On the other end I had about 3"-4" of usable pipe that fits onto the tank itself. Since all I needed was the curved part, I took a ride to a parts store and picked up 2' of the widest diameter fuel filler hose they had - 1 1/4" I believe. The same size you find on the pumps at gas stations. I also got 2' of 1/2" fuel hose for the breather and some clamps.
I trimmed the pipe remains and cleaned them, measured length for the curved bits and started to put it all together with help from my roommate. LOTS of elbow grease, grunting and cursing later and had a new filler neck. Installed, it sits about an inch higher than stock, but the door closes and locks and over 2 years later, still no leaks or further rust. Parts cost : around $20 with the clamps. We did have to lube the metal parts and stretch the hose diameter a bit to get the hose over top and clamped on but totally worth it.
Unfortunately I didn't think to take pics while we were putting it together, but I am replacing the rear shocks this week and can snap a few of the complete unit if anyone's interested.
Depending on how far gone your filler is, you may be able to get away with less hose than I did. Also useful would be access to a shop & hoist where you can work on it much easier. A tool to help shrink the pipe diameter a bit would make mating the hose to pipe end easier as well, but not a must.
When cost is a big issue, this solution is well worth trying.