Depends on what's "affordable," of course, but I think there are some in almost any price range. I always recommend people go for common ones. Can be just as attractive/interesting, but often much cheaper. ROMAN IMPERIAL... Price according to condition, but for $20-80, I never tire of the common Gordian III, Philip I or Trajan Decius silver Antoninianus or double-denarius. Big, impressive silver coins. Bit more now, but in 2012, these were $20.33 each from VAuctions. (I suspect VAuctions had no auction fee/buyer's premium back then, but not certain.) Auction 291, Lot 254 I also think the later Antoniniani -- Billon / Silvered AE ones -- are a great bargain. Especially Probus, who has really cool looking obverse types with helmets, shields, spears and so on. These were $47.20 each after buyer premium (CNG 483, 454, w/ rev. photos shown), but really nice ones can be found cheaper than that with a little patience: ROMAN PROVINCIAL... Big Roman Provincial bronzes can also be very impressive but affordable. (A nice substitute for the Imperial Sestertius, which is usually pricier.) I got these in a group lot for about $13 each with a bunch of other large RPC AE (and smaller ones, which were actually the much more valuable) like the ones below. (Some of them are better; these are just two I've photographed.) Both are common types but scarcer variants, and now on RPC online (same for many from that group), the bottom as the primary illustration for its type: RPC VIII (Temp.) 7845, specimen 9 RPC IX (Temp.) 20, spec. 3 (only illustrated) GREEK... If affordable goes up to maybe a couple hundred dollars, the really big late Hellenistic Tetradrachms of Alexander the Great are my suggestion. This one was $114 in 2019 (75 GBP + 20%) at Naville Numismatics 53, 73. It's 35mm! That's big for any coin -- but especially impressive, to have a silver coin that big in hand. (Unfortunately I don't seem to have a coin-in-hand photo to capture it, but that's 50% broader than many of the thicker, 23mm Tetradrachms -- or 24mm or 25mm -- that were struck closer to his "lifetime.")
