This is not the cheapest option, but it's a really good option that isn't ridiculously expensive. Having an all metal BB that is threaded together in the middle means you're much less likely to get creaks etc.

https://wheelsmfg.com/...s-shimano-black.html

(Keep in mind the note on the page about 24mm vs 24.07mm though -- MegaExo cranks can be either one and your BB has to match exactly!)

Will this not work just fine?

http://www.wiggle.co.uk/...t-30-to-bsa-adapter/

It should work I think. I'd bet it will at very least creak like crazy, and might be a bit squishy, but who knows?

I use these and have no trouble with them. It's on my CX/commuter bike though so I'm not putting out any big power efforts on them.



http://www.chainreactioncycles.com/...ee085-/rp-prod129801

 
Here's a cheap option:


https://www.aliexpress.com/...mp;priceBeautifyAB=1

Here's the option I would go with:
I like that it has a threaded component to it. This would make installation (and removal) a lot easier.

https://www.aliexpress.com/...mp;priceBeautifyAB=1

For real? I don't want to go through it if it's going to creak at all. Then I'd rather get a new crankset.

So I'm not sure what to tell you. In my experience, press fit BB's suck, and the basic plastic press-in types will eventually creak pretty much no matter what you do. Maybe someone else knows some secrets I don't but I've had some pretty good mechanics work on mine, and they all eventually creaked. I've found the threaded metal ones like the one I linked much better.

To be clear: the crank is not really the issue. Even a PF30 crank with a plastic press-in PF30 BB will eventually creak (again, in my experience). I've replaced all my BBs with the threaded together type, and I'm now happy. The plastic adaptor you were looking at has the additional problem that it kind of has two spots where it could creak. It's a plastic press-in BB so the interface with the BB could creak, but it also has plastic threads for your BSA BB. I can't really imagine getting the BB threaded on very tight (the BB will spin), but who knows. Both spots could spell trouble. That's just a guess though.

Anyway, for $75 you can have an awesome all metal BB that screws together super tight, will fit your existing crank, and should last the life of the bike, so it's not like the solution is all that hard or expensive. (Actually I think you can get that BB on Amazon for like $50.) You should probably ask your favorite mechanic for advice before listening to me though. Also, if you do go that way, be sure you don't have a funky Mega EXO 24.07mm crank or you need different bearings.

So is it better to get a whole new crank or can I manage to get this working with a 25-ish dollar part? Obviously I would not want it to creak on a triathlon bike where the psychological factor of a creak can mean a difference in performance.

probably not the cheapest, but works well
https://shop.fullspeedahead.com/en/type/bottom-brackets-spares/megaexo-and-24omm-spindle-cranks-to-pf30-frame

PF30 was a horrible design. I am on my 3rd warranty frame and Specialized's solution was to bond real metal cups in the frame. Just fix it with the Wheels solution and move on.