I use an Excel book with multiple tabs to track just about every imaginable data point, all of it organized to help with ongoing research. Some machines have hyperlinks that open a corresponding Word file containing more detailed information about the specific history of that machine and its lineage. 

While I do keep a column for what each machine cost me, I don't understand how current market value can be logged: In my area - as an example - there are multiple 5-series Smith-Corona models for sale at the moment (similar make, colour, condition), and they range in asking price from $70 to $285 CAD. Current value varies wildly because it's dependent on each individual seller and buyer, and to a lesser extent location. A seller can be someone flipping typewriters for profit ($285) versus an original owner (or relative working from a spreadsheet) who just wants to clear house ($70). And you know those who flip machines more than likely pay less than $50 on average for their inventory. I rarely break that amount - regardless of make/model.

As for the travel cases, I simply use hang tags on the handles that identify the make, model, serial number and my own database number for the typewriter to quickly find it in the spreadsheet. Initially I used rubber bands to hold the tag to the case, but then discovered that the quality of the rubber was extremely poor and unreliable, so I've since switched to butcher's twine.  

My biggest issue with inventory management is storage. At the moment my typewriters are spread across multiple places in the house and garage. This sometimes makes locating a specific machine a real chore. I am hoping to clear out a lot of stuff this upcoming year and have my entire collection in one room, sorted by make. This would also be ideal if someone other than myself would have to sell them off.