Perhaps some sort of "bulk buy" system could help. Basically, the price per item remains the same at lower quantities, but if you buy a significant number (like 100, as an example) then the price per item decreases by some amount (this could probably stack for every milestone hit, down to sell price or near sell price as a minimum per item price). For casual builders like me who aren't looking to make extravagant builds, the cost remains the same for the few items I really want. For people making giant projects (which usually include an absurd number of canvas items) the cost is drastically reduced; the amount of items being bought at once is what will jack up the price.
For example, let's say I'm looking to buy just one canvas: that's 1,000 Units I need to shell out. That's not much, and I get exactly what I want. Now suppose I want to make an art gallery with a large number of exhibits. Nothing major, but now I want 100 canvases. Right now, that would be a hefty 100,000 Units! Let's say that a bulk buy is in place, with every 100 canvases purchased together reducing the cost by 10%. Now, each canvas item is 100 Units cheaper, or 900 Units per canvas item; the total cost is now only 90,000 Units. A 10,000 Unit saving is quite notable, but casual builders won't typically need to have 100 canvases on hand. Similarly, we could go up to a 50% discount at 500 canvases if we wanted to start some cool project. This would give us the actual sell price of 500 Units per canvas, leading to a price of only 250,000 compared to 500,000. It's still quite the investment to make, but you do get your money back if you decide to sell.