Another opinion: I haven't closely examined the 150 market for years and may be off base, but while $23K isn't really outrageous for the above average 150, it does seem a little on the high side for an "average" 150.
Some things I considered in the pricing but generally avoided when I bought my first 150 include:
A "fresh" engine overhaul. If the engine is going to fail due to something missed during overhaul, it's most likely to happen early on. I'd like to see at least a couple of hundred hours on it before I'd buy it. A mid-time engine is somewhat proven, doesn't command top dollar, and still has years of life in it. However, at mid-time or sometimes less, it may be ready for a new set of cylinders. If they were changed within the last 100 hours or so, make sure they are "broken-in" with good compression numbers and normal oil consumption.
"Fresh" paint. Good paint jobs are expensive, and that will drive up the price. Less expensive "scratch and squirt" paint jobs can often look as good as factory, but can also hide corrosion problems that will be evident within a year or two. If the paint is at least a couple of years old and still looks good with no evidence at all of underlying corrosion, it was probably properly prepared for painting.
Primered interior skins! Same reason as above, only more so. Very few 150's were primered inside at the factory, and those were very likely done during assembly (150G's on floats come to mind, and I "think" Aerobats, too). It is almost impossible to properly prepare all of the interior surfaces of an airplane for painting once assembled. I don't like it for the simple reason that I can't see what's hidden and growing under it.
Corrosion treatments (Corrosion X, ACF-50, etc.). I got a good price on my first 150 because the paint was old, faded, worn and chalky. The airplane needed paint badly, but was otherwise a very solid airplane (the most important thing). I planned on painting the bird, but if it had been corrosion treated within the last 2 or 3 years, I wouldn't have bought it. Corrosion treatments "weep" through skin overlaps and openings for years and make it impossible for new paint to stick properly. Again, corrosion treatments can hide previous corrosion, similar to primers and paint, and are not good for control cables and pulleys, etc., in that they attract and hold dirt and grit.
Broken or worn plastic components, including interior. While somewhat expensive for what you get, all interior and exterior plastic on a 150/152 can be replaced without a second mortgage. Much of it can be repaired and repainted, and some exterior pieces are available in much more durable fiberglass. The most important interior piece is the panel cover that the engine controls mount through. It is difficult and expensive to replace in both materials and labor, as all of the engine controls must first be removed.
Know what you're buying! I got lucky! Something that I should have done but didn't is pay for an annual inspection (much better than a pre-buy at near the same price). Work this out with the seller. You pay for ONLY the inspection.... NO repairs should be done during the inspection, but a list of discrepancies (if any) is generated for you and the seller to examine. The seller generally pays for any airworthiness repairs needed (it's still his/her airplane), but any and all repairs needed can be a haggling point to be considered in the selling price, and if you buy the plane the annual is good for another full year! Remember, only airworthiness gripes ground the plane! None of the cosmetics or non-airworthiness related discrepancies have to be repaired for an annual sign-off, and can be disregarded at your discretion!
This is not a comprehensive list (I'd have to write a book) and some will disagree with me on some items.
