I think that's the issue with most of us, actually. You're in good company!
Think long and hard about Biollante. I really think the X-Plus figure is the pinnacle for Biollante, but the Bandai one is still enjoyable. There's a huge price difference, though. Bandai one will run you around $100 or so, higher for the box, while X-Plus will be closer to $600. To be honest, though, I don't see Biollante's price rising in the next year or so. Anyone else think I'm crazy on that? It's just so high-value already, most of what you're gonna find is collector's dumping them for cash. Any retailers still holding stock will want them gone. I just don't see it's price increasing any more for a while.
For your list, start working your ass off and save, save, save!
Ebirah - shoot for X-Plus. If not, the Marmit is a good substitute.
Kamacuras - go with Marusan FW version, I think? The Bandai just kinda looks broken no matter what I try. ~$80-120
Kumonga - I looooove the Marusan! ~$100 price
Gorosaurus - X-Plus, Bandai Museum, Bandai 92 in order of my love and expected value.
Megaguirus - is there much difference in price on the theater and blister versions, these days? Actually, you can pick up the whole blister pack for $100, I think.
Dagarah - box/blister from Bandai. Comes with an Aqua Mothra and some... crap... Neat figure, but I have little love for Mothra.
Monster X and Keizer - I don't know what these run these days. Not really keen on Keizer at all, so he would be at the bottom of the list for me. Monster X is a kinda neat figure.
Baragon 02 - love him! Go with the classic and skip on the theater and event variants. Very nice and inexpensive figure.
Orga and G2k - if you can afford the, I'd go for the battle-sound set. But the Bandais are also good. The Albatross Orga polystone is actually pretty cool, too.
Y-MSF - I think Tempting Toyz is always there, so between them and all the other guys, you shouldn't have trouble finding them. Tony will likely have higher prices, though. Just a heads up.
Spacegodzilla (Movie monster series) - *cough* 10" Trendmasters *cough* Okay, okay. The original SpaceG repaint might be a little harder to find, but shouldn't be killer in cost.
Godzilla '74, Titianasaurus, and Mechagodzilla '75 - Nice set, but likely a little more expensive. Keep an eye out for the Toy's Dream box set. Titano has a better paint job and G is in there, too. Mechagodzilla should be found in bulk, but I imagine he'll be above $50. Am I way the hell off on that?
Mogera and King Seesar (Movie monster series) - Good luck. These were the last two of the MM series I was able to find, and that was just last year! Best paint jobs of the lot, so they run between $65 and $100 each.
Burning G (MM) - I think this is a good choice. Shouldn't be too expensive and is a great figure. The original 8" is neat, but the orange is dull by comparison. If you can find and afford it, the 8" Meltdown is a great choice, as well.
As a bigger collector, I gotta tell ya, don't get too attached to boxes. This is heresy, but I'm so sick of the damn boxes! You'll save space and money if you throw off that chain.
Your biggest enemy will be the X-Plus stuff. It's so freaking awesome! But otherwise, I think finding your Bandais is very possible, even on a small budget. And if not, some of the 8" variants occasionally go for less than the 6" ones! If tags aren't a priority, things should go even better. If you can pull together $500, I think you'll be very, very happy. Even still, $300 would be a great haul. Kinda hope the Legendary movie doesn't get a date before I can attend another G-Fest to keep prices low. Though, I think a G-Fest with a new American film just out or coming up would be a glorious convention, indeed.