The first thing to bear in mind with overture is that you are using a "search term" suggestion tool, not a domain suggestion tool. It treats some searches as phrases, others as sets of words that are interchangeable. It does not distinguish between singular and plural. It corrects and alters spelling.
"horse to buy.com" is a search term that the tool suggests based on your keywords "buy.com" and/or "horse". You will get no typins on "horsetobuy.com" with or without hyphens
Second thing to bear in mind: "xxx OVT" is a marketing term fabricated by domain speculators and used almost exclusively on forums such as this. It has no direct bearing on traffic. It's frequently confused (often intentionally) with OVT w/ EXT, which has bearing on traffic.
A high overture simply means that you have picked a phrase similar to what many people search on.
For example: "security online" has an overture of 274K. However "securityonline" has an overture of 0, as does "securityonline.com"
What you can deduce from this is that "security" and "online" are a nice pair of keywords. You'd probably want to use them your meta tags. The domain names using these are certainly worth something as two word names. They would probably gets a few typins, but nothing substantial.
On the other end of the scale you have "price line" with an overture of 45K. For "priceline" you have an overture of 204K, and for "priceline.com" you have an overture of 269k.
What you see here is that "price line" is not a particularly common phrase. What's happened is a company has come along, branded itself as priceline.com, and generated massive brand awareness. This is a name with millions of typins.
People generally only type in certain things, all of which have typein potential
Names that they know such as Hotmail.com.
Names that are branded as dotcom, such as China.com.
Single words and common phrases
Misspellings of the above
If a name has no overture with extension there are a few guidelines you can use to give you a right idea of what sort of typeins you might get.
With a generic phrase like "nice weather we're having" it would generally mean nothing no matter what overture it had. The exception is certain catch phrases, song titles, etc.
A short generic 2-3 word name with >50k overture should expect some typeins, more if it still has an overture with the spaces removed.
Made up words that have any overture will get typeins.
If you're interested in typeins it's worth experimenting. So you blew $7.95 on madonnaoftherocks.com. In the process you learned something. Next time you'll do better.
The thing is that with reg fees and hosting as low as they are maintaining a name is cheap. If you can park it at a PPC search engine and make $1/year more than it costs then you are ahead of the game. Do this with a few thousand domains and you have a nice little zero maintenance earner.