Welcome to Numista jhw!
Not sure where you're coming from but I can honestly tell you (from my neck of the woods) if I were to walk into my LCS with my greensheet & started to haggle with the owner based on my greensheet tables, I am pretty certain he would tell me to open up my own shop first & then get back to him to haggle.
I actually bought a Standard Catalogue of World Paper Money (or SCWPM), the thick tome which sells for around $99 (with 30% discount) & have rarely use it. Found it to be pretty dated & used it mostly as a reference point. Most of the approximate BV's were very generous (market prices weren't as high as the SCWPM tables) which was odd. I have found eBay & other online sources equally or more accurate as a source of info but perhaps that was my personal experience when I started & the market was somewhat depressed (around 2015). It just takes time to familiarize yourself with the asking prices (just like it takes time to accurately grade). Pricing World notes is definitely a challenge these days & I'm sure its a big turn off for many who might be curious about the hobby (there's no current catalogue other than the greensheet, if you can get your hands on a copy).
Ten (or 15) years ago it wouldn't even be an issue (to study greensheet) because most banknotes you bought on eBay (or most online stores/outlets) were wholesale (or nearly WS) priced. Everything was quite cheap online. Today, it has gotten a little more expensive but if you're buying from a reputable seller (ebay) then you will always end up doing a little better then had you walked into a brick & mortar shop (esp on an auction listing).
@blue-m sums it up for me: do your homework & you will find the hobby rich & rewarding (but maybe not be as $ rewarding as you'd like). I don't look at the hobby as a means to invest. If you wish to invest, there's far better places to park your money.
I tend to look for PowerSellers (they can sell most reasonably) & sellers who list Auction style unless the BIN (Buy It Now) is indeed flat-out the lowest I've seen. But I always inspect the note well (many sellers don't count foxing, spots & counting folds when assigning UNC as a blanket grade). I try for UNC or upper grades unless its something pretty tough (than I'll settle for VF35 or better). Some collectors turn to Track & Price but I am a bit gun-shy on a monthly subscription & just try my luck on auctions with some of the few more pricey notes I go for.
I'm not an expert on Costa Rican banknotes, but “r” in the number could mean it's a replica (like restrike coin).
“R” stands for replacement - in most paper catalogues but in terms of that PS121 1 Colone remainder, Numista actually explains how the American Banknote Co auctioned off 61,000 examples in 1996. They're pretty common & were never issued. If you like banknotes that never circulated, then you'd be better off going after specimens.
Remainders are left-over banknotes typically unsigned & without serial numbers (or other overprints).