more robust pricing system

Currently, prices on items are expressed as a string (e.g. `"30 dUSD"`). The intention was to allow for other currencies besides dUSD, but there are some less brittle ways this could be handled.

In the simplest case, split price into amount as a number and currency as a string. However, so far dUSD is the only "currency", so maybe we're inventing a solution to a problem that doesn't exist. In such a case, maybe it would make sense to just use a number and assume dUSD (at least for now).