I'm sorry, we're not ready to go this down road yet. It seems like a simple change—one field on one page—but it's more complicated than that.
Fuelly has always been focused on tracking fuel economy only. So when people read and use the site people know that's what we're discussing. So on vehicle pages we have labels like "Total Spent" and "Price/Mile" and everyone understands that those numbers relate only to fuel consumption.
If we start tracking more than just fuel, we'll need to make that clear across the site. So we'd need to change "Total Spent" to "Total Spent on Fuel", and "Price/Mile" to "Price/Mile for Fuel". Then we'll need to add fields for "Price/Mile All Expenses", "Other Expenses Total", etc. That probably sounds awesome for some people, terrible for some people, and just ok for others. No matter which camp you're in, it's a significant change for the site that will require some major retooling under the hood.
We're not completely opposed to the idea of Fuelly becoming a "total cost of ownership" application instead of a "track your fuel economy" application, but it is a big shift that we need to consider carefully. Our mantra has always been: "keep things as simple as possible." We like tools that serve a single purpose and do it well better than tools that try to do everything and miss the mark.
Once we have a price value on notes and people are used to using Fuelly for more than fuel, other feature requests will naturally follow. The price field won't be enough to make this change. And that's ok too, but we're not currently prepared to devote the time to giving Fuelly this sort of personality change. Like I said, it's something we're seriously discussing but we haven't made the decision yet.
In the meantime, there are applications out there that track more aspects of your vehicle than fuel consumption. It could be that you need a tool that does more than just fuel-ups so that might be worth looking into.