enochian, I am going to blow your mind right now. You see, I am cheap and only buy raw coins. Sorry for the image spam, but I want to show you want you can get for under $10. You mentioned you couldn't find any uncirculated Mercs for less than $15. I suspect you are looking at "buy-it-nows". I suggest you go after auctions to get the best bang for your buck, or ten.. Patience will nab you some real beauties for a great price. Not one of these were over $10 (delivered). Most were $6. Except for the Franklin's at the end, those were $18. These were $18.
The 1932S may have received a 65, but the seller cannot believe that particular coin is worth close to what they are asking?
Yes, if I see a twice as nice coin for twice the money would be a AU/MS coin for $2.60. I have other IHCs I got for a buck that are VF. Maybe get an high AU for $2?
The Standard Catalog of World Coins published by Krause. I would check ebay and see what various editions are going for. They have volumes for 17, 18, 19, 20, and 21st century. 20th century would probably serve you best at first if you are getting inexpensive world coins.
I would rather have 5 MS60s for $2 each because a couple of my dollar coins are XF I believe so MS could be two dollars.
Buy 4 decent "junk silver" Mercury dimes. Put a free ad on CraigsList offering to trade them for the best 2 silver quarters offered. Offer to trade the 2 quarters for the best 6 Mercury dimes offered. Repeat. Repeat. Repeat. In the meantime, silver might POP over $35, and for all practical purposes, you will then pick up an extra coin free. Think of this process as your own personal scavenger hunt.
I have a good idea, just buy what you want. It's $10, not like you are going to lose much. Just buy a coin you want on Ebay with a really bad picture and hope for the best. Everyone is offering up what they would buy for $10 but it is not our $10 so buy what you like. You can just put it in the bank and if a deal comes up, buy it.
What I am trying to communicate is buy the best grade of the single coin you can for the $10 instead of splitting it up into multiple coins of lesser grades. This is my general rule. However, I have purchased a slabbed 64 coin when a few dollars more would have gotten me a 66-67 because I wanted a 64 grade for comparison and research purposes. If I am solely trying to put together a set, then I try to figure out a basic budget and purchase coins at a "set grade" instead of purchasing the best single coin I can at the moment (there are exceptions). Perhaps this is your suggestion, start an IHC set and complete it on a cheap budget. If this is the case, I think it is a good idea. But, again, if the OP's intent is a single purchase, then good numismatic literature or a single coin in the best grade $10 will buy. For a single coin, Mercury dimes have a cool obverse, but the bicentennial quarter has a very singular reverse celebrating the country's 200th birthday.