I have lots of (mostly development) sites, but use almost no file storage. IMHO, paying a multisite per-site fee equivalent to the single-site fee is rather ridiculous, because I incur essentially zero additional cost for each additional multisite network site.. It only exists currently because the managed-backup market is so immature, and no competition has arisen to challenge it.

As Donnacha points out: once a competitor comes up with a more appropriate model, that competitor will make a killing.

The primary differences between single-site and multisite installs:

1. Database size: more tables, larger DB size for multisite
2. File storage: potential file-storage for /wp-content/blogs.dir than for /wp-content/uploads.

The costs incurred for #1 ought to be essentially negligible, and yet they constitute the bulk of the value for the end user. The costs incurred for #2 are directly proportional to the number of sites supported, but in bulk, those costs are pretty low.

So, what I would like to see:

1. Flat-rate annual subscription cost, for single site or multisite network (e.g. $25/yr, $50/yr, whatever...)
2. Per-usage cost for file storage, charged per-GB of storage

I would pay for this service/pricing model in a heartbeat. I daresay that lots of people would pay for this service/pricing model in a heartbeat.