Ancient coins can be very small, like the size of a flattened pea, to about the size of a half dollar. I have one from Alexander The Great that's so small it looks like someone lost a silver filling - cool how old it is but not much to look at.
In general they are made of bronze (dirt cheap), silver (more expensive, still affordable), or gold (expensive). Also, the sharper the detail, the more they fetch. I have a couple that are large, silver, and have moderately good detail still and they might fetch $1500 or so, I think. But most are very affordable.
From the ebay link I shared you can get five or ten coin lots for under $40. You can get a single coin for under $10. Those tend to be smaller and worn. Most of mine are in that range.
If you want one just for the "wow" feeling, there's no reason not to get one, they can be had for not much money.
The one for sale right now is asking for $2,700. Yikes!