The prices are going up. The popular bars I listed are very aware of what Americans are willing to pay.
Breweries are starting to charge more but the cheapest places to buy Lambic are at the breweries or beer stores that are off the beaten path.
If you're looking for "rare", you're going to pay appropriately. There are some cafes not spoken of and not really listed in many places and they're charging pretty fair prices but honestly, most people that know about them aren't talking about them. Lots of places bought Lambic throughout the 80s and 90s as demand was low and now have quite a stockpile.
Expect to pay 5 Euros for current vintage Gueuze / Kriek 37.5s @ bars. $25-$50 gets you the older stuff. $100 euros is generally reserved for the high end 'whalez' but a lot of those bottles are long gone and menus just haven't been updated.
If I'm in Belgium for 4 days of straight up drinking and unrelated, I have a rental car so getting around is way easier (I'm not driving drunk), I can probably spend 150 Euros a day on beer with a mix of older and young beers for 2 people.
Here's a 2012 receipt from a beer- store https://farm8.staticflickr.com/7073/7163802913_9a0b7d8b91_b.jpg
Add about 30% to these prices now on the Cantillon stuff and more difficult to find all of these in one place but that should help you a bit as well.