I doubt if the remote site is looking at your MAC address.
I would be surprised if it were using your IP address.
It may be using a cookie to detect a repeat visitor.
The more likely scenario is that it's like some companies I deal with (the local railway company is one) where they offer so many seats at "not a lot", then some at "a bit more" and so on, until on the day of travel you pay the full fare.
If I book to go to London in a month's time, I can probably get a ticket for £8 to £12. If I book for next week, it will be around £20. If I pitch up at the station on the day it will be around £60.
If the memsahib sees a price increase from site visit to site visit, I would suggest it's probably to do with the date and time of the visit.