As for exchange rates between various coins, I'm going to say I disagree with Snorkack. Sure it's quaint to us to have non-decimal system but you're trying to build a believable world, not a convenient world. Historically, decimal currencies only began to be used in the 1700s and the British started using decimal currency only 40 years ago. Until then there were coins like shillings, guineas, sixpence, crowns, sovereigns, etc. The non-decimal denominations, just like non-decimal measures for time (hours, minutes, weeks, months), weight, length and so on, they existed because they made most sense in their time, not because people never thought to use a decimal system.
In the case of coinage, the reason was that there were more divisors when the base 12 and base 20 were used rather than base 10. When you have 10 coins, you can only divide it 2 ways and 5 ways. If you have 12, you can divide it 2, 3, 4 and 6 ways. Similarly if you have 20, you can divide it 2, 4, 5, and 10 ways. So you may want to rethink having a 50:12:1 ratio, maybe 60:12:1 would make more sense or even 144:12:1 if you like things consistent and if coins are automatically converted to higher denominations in your system.
