what size apo

i currently have an orion xt10i which i enjoy but have been giving more and more thought to purchasing an apo/type to go along with my dob
the sizes i have been looking at range from 80mm to about 110mm, based on the price range factor.
the 80mm, depending on brand are in the $600 price range for the tube only and the 102-110mm i been looking at go up to about $2,000.
there seems to be alot of so called apo's out there right now for around $1,000 in the 4 inch range.
is there a best size for the dollar
and is there a big difference in doublet vs triplet?
thanks for your help
ps i also understand that with these i will of course need a tripod, diag, finder scope.

The costs of making a refractor goes approximately by the cube of the diameter. Therefore, for any particular performance level, if an 80mm costs $600, then the same performance level in a 100mm will cost (100/80)^3 x $600 = $1171. A 120mm will cost (120/80)^3 x $600 = $2025 and a 130mm will cost (130/80)^3 x 600 = $2562.

Reason for the power of 3 of the cost is that the costs of glass, metal, machining etc goes up by the volume of material used and volume goes up by the 3rd power of diameter. If any scope lies above or below these numbers, it will probably be because the performance is either higher or lower in some fashion. Whether you need higher or lower performance is something you must judge.

Rolando

Best size fto dollar ratio has got to be with some of the smaller scopes. I would say an 80mm is the best value and would complement your XT10i quite well---easy to transport, easy to setup, no collimation required, minimal cool-down time, and, of course, dramatically dimmer views. 80mm won't get you the best scope, but it proabably will get you the best value and you'll find you use it more than you might think. The Orion 80ED, for example, is inexpensive and (because of its small aperture) is very nearly apochromatic. Keep in mind that as your aperture increases, if your focal ratio stays the same, color error increases too--another advantage for smaller scopes.