The general answer is that like many space collectibles, pins with the highest value are typically older and/or more scarce, or have a special significance or history.
The number of different pins and the quantities they were produced both grew with the program. Some Mercury and Gemini pins today may number in the low hundreds, while many Apollo pins are in the thousands (or more).
High-end NASA-issued pins such as flown pins, Silver Snoopys, astronaut pins and the like are highly-collectible and carry the most value. Expect $500-$1,000+
Some NASA and contractor pins from Mercury and Gemini can go anywhere from $10 to $75 typically.
The gold "launch team" pins for Apollo missions had been on eBay pretty commonly for years although oddly I haven't seen many recently. They are in the $10-$30 range now and were sold to anyone who bought them in Cocoa Beach and Merritt Island gift shops into the 1970s.
Modern shuttle-era crew and payload pins rarely have a value beyond three or four dollars simply because of the numbers they were made in.
Another interesting area is pins relating to such early programs as Surveyor, Lunar Orbiter, Ranger, Mariner, etc. These are uncommon but likely of highest value to a specialist (most still under $100 I'd say).
One pin of note is that made for John Glenn's "launch crew"; a beautiful piece with a value in the low thousands; only a handful were made and I believe only two have ever been on the market.
That's my opinion as a 40+ year US collector; others may have another view. Hope that helps.