There are others but these are four pictured below that won't break the bank...If purchased one at the time.
I've been doing a lot of talking about the waste of many modern pieces. But with all due respect there are a few on the flip-side that for the average collector these ARE as I said, MUST HAVE PIECES that will not destroy one's pocketbook.
Any reverse proof coin in silver or gold, and if the funds are available, the Reverse Proof $50 Gold Buffalo which by the way is the only coin that has ever been made in the .9999 fine 24K category, with this finish to my knowledge. It is one of the 3-4 not so cheap pieces and are one's I don't have, but are coins I really want to pick up and leave raw in their gorgeous original Govt. packaging that they come in. There is another $50 Gold Eagle in reverse proof that is second on my want list, and it too, costs a pretty penny to procure. But without a doubt these two big gold coins and the others will one day make the person that purchased them, a very happy camper indeed. In the OGP these will be in 100 years probably about like having an original Pan Pac Set from the 1915 era in its original Govt. packaging at today's pricing which is very steep.
Those that can afford these two coins and the others that come with the Gold Eagle in their respective sets, as I think the Buffalo is a one coin set which is expensive enough by itself. But again it is a coin that has never been done before, and most likely will never be minted again. And most importantly, minted in ultra low numbers.
But these modern coins along with the 1907 High Relief Re-Strike $50 Gold Eagle are 3 expensive moderns that come to mind that each and every year they have gone up in value, and are pieces that you will never lose on. Even more so while the price of gold is much lower than it use to be when these coins were produced, and of course because of the economy we are in, at present. I have seen very good deals on eBay for all of the coins mentioned in this post.
But if the purchase of these are most definitely out of the question, money wise, at least take your time and if you haven't already done so already, pick up the 4-coins pictured below. And if at all possible get all four in MS & Proof 70. Forget the MS & Proof 69's if you can. Especially the 2013 Enhanced Finish Silver Eagle in MS-70. This coin you must have in MS-70. Of the 4 coins below, if you can only afford one of them, get the "Enhanced Version" Silver Eagle in MS-70. You will not be sorry, as it will surpass in worth, possibly all three of the others combined one day.
Universal Coin & Bullion had the two coin set of the 2013 in the Reverse Proof & the Enhanced Finish for $299 for the pair in the Star "Black Label Holders" in MS & Proof 70, just a couple months ago, and they still may have some left. They had the 69's too, but forget about them if you possibly are able to.
If you don't purchase another coin in your life, try to get these two "Black Label" coins in 70. You will not be sorry!