It all depends on what you're going for. The original supplemental overlay pack for the Flashback (2014/2015) was a STEAL at $15, especially since overlays for orphan (INTV Corp) games were included. Prior to that set, single pairs of Orphan Overlays by Psycho Stormtrooper were selling for an average of $6-8 with some (Diner) occasionally selling for up to $25
There was a recent price surge where a few sets (<10) sold between $100-300, however the most recent sale on eBay was $52...still high, but it's hard to beat $1/pair under any conditions. Homebrewers can chime in if I'm way off.
If you emulate, use a multicart, or only have the loose carts and just want overlays, you have a few options:
1. Originals on eBay: Individually, you probably won't find any for less than $2/pair ($0.99 + $1.00 shipping). You might find a seller with a bunch who is cool with combining shipping. I occasionally see bigger (more affordable) lots on eBay, but it's hit or miss.
2. Make your own: Much more work and potentially expensive if you want quality.
3. Ask around here or post in the wanted section of the marketplace. Maybe someone will throw you a bone.
4. Honestly, most games don't require them and as you learn the game you stop looking anyway. There are pics of the overlays in the manuals and ample online sources where you can reference/print them. However, some games do use the keypad more heavily. You may want actual overlays for those (D&D, Tron, Utopia, Night Stalker, Sea Battle, Sub Hunt, etc), but they'll take a beating (and are harder to find in good condition).
If you're just putting out a feeler for a cheap set, good luck. You never know until you ask. If you're willing to put some work into it, read up and try making some...or put some time into aggressively hunting for them (ebay, retro stores/sites, etc). How many games are we talking? 10 or 125? The less you need, I'd recommend buying originals on eBay. Want a complete set? Tougher task. Either way, good luck!
Edited by JasonlikesINTV, Thu Jun 22, 2017 6:18 PM.