Your price range makes things a little difficult.
If we (properly) define a ‘macro’ lens as one that will allow magnification up to 1:1 (image size = subject size), the cheapest you’ll find new is the Canon 50/2.8 at $269 from B&H. Yes, I know the Canon 50/2.5 is cheaper ($240), but it only goes to 1:2 without the ‘life-size adapter’, which costs about the same as it does ($230)! And yes, Vivitar and Phoenix also make macro lenses, but let’s not go there.
If your price is absolute, try for a used one at Fred Miranda (look it up on Google) or eBay. You might find the lens quoted for less by other dealers, but if you find one for $200 or less new, watch out! If a deal’s too good to be true, it’s probably not true!
There are some other ways to get to the macro range, cheaper than a dedicated lens. Someone’s already mentioned extension tubes – I’d suggest the Kenko 3-tube set ($169 from B&H), or the cheapest way would be a reversing ring. Do a Google search on this – the rings are cheap ($10? $15?), but you’ll need to know what lenses he has for the sizing.
There are several other ways to get macro shots, but by far the most convenient is an actual macro lens, such as the Canon 50/2.8.
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BJCP National