This isn't really true in the market today. 100G switches have emerged at roughly the same cost per port as 40G from a couple of years ago and, indeed, can often accept both 40 and 100 gig optics. Even at list price the cost per port for 100G switching has been under $1K for more than a year.

As a result it's actually fairly common to find new DC fabrics (read: inter-switch connections, not end hosts) being built with 100G because there's no significant economic disadvantage to doing so. That said, the pricing for inter-site 100G is still high enough that it hasn't commonly made its way to smaller organizations.