  Cost data for ground-based telescopes of the last century are analyzed for
trends in the relationship between aperture size and cost. We find that for
apertures built prior to 1980, costs scaled as aperture size to the 2.8 power,
which is consistent with the previous finding of Meinel (1978). After 1980,
`traditional' monolithic mirror telescope costs have scaled as aperture to the
2.5 power. The large multiple mirror telescopes built or in construction during
this time period (Keck, LBT, GTC) appear to deviate from this relationship with
significant cost savings as a result, although it is unclear what power law
such structures follow. We discuss the implications of the current
cost-aperture size data on the proposed large telescope projects of the next
ten to twenty years. Structures that naturally tend towards the 2.0 power in
the cost-aperture relationship will be the favorable choice for future
extremely large apertures; our expectation is that space-based structures will
ultimately gain economic advantage over ground-based ones.

Instrumentation and Methods for Astrophysics