The spire is a great place to "trade" large amounts of goods for diamonds. If you are spending more diamonds than you are making, then you need to learn how to negotiate (or fight, but I negotiate, usually). And if you enter negotiation with the thought that you don't want to part with many goods, then you also need to learn to negotiate. It costs goods. You aren't going to get diamonds for nothing.
If Inno were to offer a sale of 50 diamonds for, say, 500,000 (or whatever your level considers a lot) of your boost, a lot of people would jump at the chance to buy. And yet they don't want to spend 500,000 goods to complete the first level, even if their group ALWAYS makes it to the bronze level and they know they'll get 50 diamonds at the end. I really don't understand that. You're basically trading goods for diamonds, and you're also getting a whole bunch of bonus freebies along the way. Sounds like a great idea to me.