Ask YC: Money from your users.

Ok, so I know that a lot of you are developing free services, but for those of you that aren't. Let's say that you rent digital tools (we'll call them sprockets) on a per-month basis; different sprockets have different line item costs, and you can buy a machine that has a set number of sprockets x y and z.

This is the pricing structure that I'm planning on (the numbers are pretty arbitrary)

Machine 0 - build your own, at line item value.

Machine 1 (possibly free-to-very low cost) has 2 x sprockets 2 y sprockets and 4 z sprockets

Machine 2 has 2x the number of default sprockets, at 90% of the price of buying them at their line-item value.

with maybe 2 more machines, with different % of the line item value (more value if purchasing more sprockets)

This is my question: should users be able to add sprockets to the machine, and then be billed accordingly, or should users have to purchase a sprocket to be able to use it. Has anybody done research on what users want from this type of scenario?