As I already pointed out, it's often a case that the WIRING won't support Gigabit, and in THOSE cases the cost would be a ton higher and it's impossible to justify the extra cost for a router that will likely never run at Gigabit speeds.

 The other factor is that a lot of the time good 100MB gear can be had for PENNIES ON THE DOLLAR - in fact I was recently GIVEN a Cisco 48-port 100MB switch (it also has 2 Gigabit "backbone" ports) that the school district a friend was working for was going to just toss in the trash during "infrastructure upgrade work" they were doing this summer.
 My TOTAL cost for 48 ports of 100MB + 2 Gigabit ports? Well, it DID need a power cable - standard C13 that I have a TON of spares for sitting around - so ZERO.


 For reference, up to middle of last year I was still running machines on 10Base-2 (COAX) - the only reason I stopped is that all of the cards I had were ISA and I retired all of my old ISA machines when I moved cross-country last summer (they weren't worth loading up for the trip).
 10Base was severe overkill for what they were doing - but when I found a bunch of 3Com "trio" port cards at a Hamfest for $2 EACH in the early 1990s, and already had a bunch of BNC "T" connectors and patch cords, the price was impossible to pass up and I got something like 15-20 YEARS of usage out of most of those cards (I did have a couple of the 20ish I bought die over the years).


 But as I already pointed out, for MOST folks there is no reason to NOT go Gigabit, and pay the small premium for futureproofing.