I keep track of my expenses pretty closely using Quicken, and trips like the Loscon-SMOFCon-Tonopah trip are things that I flag as potentially charitable tax deductions on account of nearly all of it (except the time in Cottonwood AZ for the train trip) was for the purpose of promoting Westercon 74, a part of SFSFC, which is a tax-exempt charitable organization.
While business travel allows 58 cents per mile of travel and is generally adjusted upwards with inflation, the charitable mileage rate has been stuck at 14 cents per mile because it's fixed by legislation and hasn't been increased in years. It's clearly too low. Alternatively, you can deduct your actual direct expenses incurred, which is mostly fuel. (Parking charges are something you can take whether you use the mileage rate or the direct-expenses cost.)
For the November-December trip, I kept careful track of both our mileage (treating the incidental mileage from Cottonwood to Sedona separately) and how much we spent on fuel. At the end of the trip, it worked out that the mileage rate and the amount spend directly on fuel were almost exactly the same (within $20 of each other). This as much as anything shows why the 14 cents/mile legislatively-mandated charitable mileage deduction is hopelessly out of date, as it makes no allowance for wera and tear on one's vehicle.
Whether I'll actually end up with enough direct travel expenses to justify itemizing deductions on my income tax remains to be seen. In all of the years I've kept such track, there's only been one year when it was justified (2002), and inasmuch as I was reimbursed in 2003 for some of the expenses I'd incurred and deducted in 2002, I also had to learn how to report that on my taxes. (I reported the following-year reimbursement as miscellaneous taxable income and paid taxes on it.)
This, incidentally, is why I know how much money my involvement in fandom has cost me over the years. Despite what some nitwits seem to think, I don't do the volunteer work for the tax perks (even the year I took the deduction, the monetary benefit was only a few hundred dollars), but inasmuch as they are there, I always calculate them, just in case. Would I still do as much volunteer work without the potential tax deduction? Almost certainly. I have a really bad lifetime case of Volunteerism.