I would call about 3 painters and have them provide their estimates. Good thing estimates are still free these days. Here are some questions that will make you look smart and prevent them from over estimating.
Will their price change based on colors? Siding + Trim or Siding + Trim + Window Trims, or Siding + Trim + Fascia + Window + Doors
Compartmentalize their quotes. That way you can catch them where they are upmarking you.
Ask for Sherwin Williams/Kelley Moore paint grade - 200/400, etc, that will be used?
Will it be spray painted or by hand, how many coats?
Will they use a power wash to prep it? How about sanding, graining, caulking?
If your siding needs replacement, who will do that and how much if they do it?
Ask for homes where they recently completed and are currently working on. Drive by see them work
Lastly, 75% is my magic mumber that has always worked. Whatever their quote is, put your first counter offer in at 75% of their quote and dont budge over 85% of their price.. Helps establish the vendor-client relationship.
Hope this helps!!