I am not going to say that ticket prices are cheap, but nor is a flight when you turn up and go.
Given Intercity trains have unreserved carriages, I'm not sure how London to Birmingham could ever see you having no seats for the return? As long as you don't board a few minutes before departure, you'll get a seat. And, unlike a plane, if you don't get a seat - you can stand (if you don't wish to wait for the next train). Or, you could take a slower route - another option that might not be available to you by flying.
The fact is, if you need to send staff, or yourself, from Birmingham to London 60-70 times a year, it's going to be expensive FULL STOP. Getting a company car and paying all the necessary tax would cost a lot too, including fuel and parking charges (plus the extra time that driving would almost certainly take more often than not, whether using the M1 or M40, and driving around the M25 and into town as required).
HS2, with more capacity (perhaps more important than speed) can only help bring down costs further. If nothing else, the current route would become cheaper as it tries to compete.
TBH, I can't believe that given rail is probably the most economical option of transport (besides cycling, but I figure you're not going to cycle to London from Birmingham) there is even a debate at all.