How Much Are Bimetallics Worth?

I can only speak from experience about Thai coins.

The first 10 Baht bi-metallic for general circulation was issued in 1988. Although there were some mono-metallic coins issued before but as commemoratives.

Prior to the general circulation coins a 10 Baht note was used. The last ones were issued about 1991. I would imagine the vast majority of coins are still in circulation. So you're looking at a lifespan of 30+ years. A bank note would be about 3-10 years at best. The conclusion to me is that it's more cost effective to produce coins rather than notes.

As for the choice between mono- or bi- I would say that the bi- is more distinctive and not readily confused with lesser value mono- coins.

Some years ago a 2 Baht coin was introduced to slot in between a 1 and 5 Baht coin. Confusion quickly set in because they all looked the same metallic wise and a lot of store owners would mark the 2's with a felt-tip pen. The goverment subsequently issued new 2 Baht coins in a brass colour.

Having said all that a 10 Baht coin (still being issued each year) is currently worth about 32 US cents or 25 Euro cents so I can't see the Thai government producing coins that have a higher metallic value.