Rare But Cheap. Those two terms sound like a contradiction. They are not. There are opportunities for the US collector to own rare items in their collections, but at a very modest cost. Here are some things to consider.
Try finding any of the 18˘ stamps postally used on cover during the proper rate period. They do exist, but they are not common (excluding of course, First Day Covers). The 18˘ rate was quickly superseded by the 20˘ rate. The 18˘ James Hoban stamp, Scott #1935, was issued Oct 13, 1981, just 18 days before the rate changed to 20˘. Good luck finding a Scott #1935 properly used on a non-philatelic cover before Nov 1, 1981. It's tough!
The same can be said of some of today’s covers. The recent Lincoln stamps were in use a mere 91 days until the postage rate went to 44˘. How many Lincoln stamps will be properly used on cover during those 91 days and survive in collections? Many of today’s commemorative stamps see very little use on cover.
Used Special Delivery stamps are pretty easy to come by. Even the early ones are modestly price. None are rare. Try finding a special delivery stamp properly used on a postal card to pay the special delivery fee. Only a few are known used on postal cards. Finding one is like finding a needle in a haystack.
Collect plate varieties on early Bureau issued stamps. You’ll need a good magnifying glass and a sharp eye. But you can have endless hours of fun obtaining rare plate varieties on otherwise inexpensive stamps if you take the time to look for them.
Many of the back of the book areas (revenues, Offices In China, Official Seals, Local Posts, etc) have some really rare items that, in many cases, only cost a few hundred dollars or less, assuming you can find them. Their equally rare counterparts in the front of the catalog bring thousands of dollars.
The point is that there are a lot of challenging areas to collect if you put your mind to it. They don’t all require huge sums of money. I could go on for pages with more ideas, but you get the point. When you’ve reached that point where the new material for your collection trickles in, consider undertaking a new area of philately. Many of those areas are surprisingly inexpensive.