Now that you've got a price range in mind go ahead and head over to your favorite online computer store (I recommend Newegg or TigerDirect) and you'll see they have a whole category for Motherboard-CPU combo deals (the links above will take you directly to that category). Very quickly you'll notice that each combo is listed as one price. This reveals an important aspect of buying your CPU-MB as a combo: it can be difficult to tell the price each item runs for individually. So when shopping this way you need to think about the balance in price between your MB and your CPU. There are three ways you can do this:
More Expensive Motherboard, Cheaper Processor: You'll want to do this when you are concerned about durability and not too concerned about running intensive programs on your computer. If you want a computer that will run basic processes like web browsing, email, and word processing and do it for a long time, this is the way to go. Cheaper motherboards are more likely to make of low grade materials with less work being put into durability. So the balance of the price here would be around 35% MB-65%CPU.
Cheaper Motherboard, More expensive Processor: The most common case for this is when you have a mid-range computer, especially if you don't want to invest in an expensive video card which would normally take off some of the load from your CPU. The other scenario is when you are buying a higher end system or gaming rig--in this case a MB around $100-$150 might contain all the features you need leaving you open to spend more on the processor to handle all the processing you'll be doing (even with a great graphics card)
Balanced: Most people would probably think they want to keep their MB-CPU prices about balanced so that you can have both of these key components be comparable. But keep in mind the advantages of a more expensive MB or CPU so that you can get what fits your needs. "