Let's assume we are talking about collectible handguns. The same thinking applies to long arms, but IMHO handguns provide a more diverse field. So, think about the older .32 and .38 RF and CF revolvers of the circa 1870-1910 era. There is an amazing variety, most are still reasonably priced and many are well enough made to be acceptable for self defense if necessary. In some cases, one make and type of gun is high priced, while other guns by the same maker using the same system are relatively inexpensive.
Examples: H&R, Iver Johnson, small Merwin & Hulbert guns (the large guns are expensive), Shattuck, .32 Deringer revolver, Hopkins & Allen, Forehand & Wadsworth, etc, etc. Many of these are "name" brand, and not at all in the cheap "suicide special" category.
Another example: an S&W DA .32, First Model is listed at $3250 for VG, $8500 for Excellent. But the Second Model can be had for around $200 in VG.
In brief, get a copy of Flayderman and look at the "Cartridge Handguns" section. Almost anything in there has some collector interest, some obviously more than others. Also look at the Colt, S&W and Marlin sections where some models are quite low priced, especially if in less that top condition. Many collectors will advise owning only high quality guns in new or near new condition; that is good advice, but can impose a severe limit on a beginner's collection unless the beginner has reserves of cash and the goal is pure investment.
A whole area of collecting that is, IMHO, a sleeper, is the vast number of DA Colt revolvers from c. 1900 to c. 1945. Beautifully made, rugged and reliable, nice ones will not long be in the $250 range where they are today.
Jim