I think the cheap route is to either go with stacking a lens or with extension tubes. A couple of notes about both.
Stacking does present the problem of being able to control the aperture of the reversed lens. You can use manual lenses as previously stated. There are a lot of good cheap old manual focus lenses available but it is easier if you choose one with the same filter diameter as your main lens. Also, you can use an AF lens if you an old rear lens cap (the one that goes on the camera mount end) to open it. Many lens caps will automatically open the diaphragm blades when you put them on. They have a plastic that corresponds to the diagram level on the lens. When you put on the cap and turn it to lock it in place, it opens the diaphragm. If you take one of those old caps and cut out the center of it, it will still open the diaphragm but you will be able to shoot with it on the reversed lens. But it does add some extra length to the "front" of the reversed lens which at high magnifications can limit your subject distance.
Also, stacking make make focusing a bit of a pain.
Extension tubes are a much easier fix, IMHO. For $100 or so you can get a decent set of Kenko auto-extension tubes for your camera brand. These generally have three tubes that give you varying options up to a total of 12 + 20 + 36 = 68mm of total extension.
But you might not want to use them on a 180mm macro lens because that almost defeats the purpose.
Total magnification is roughly the lens focal length divided by the extension. Of course, you have to include the built in lens extension to that total.
So a 180mm lens that goes to 1:1 (1X) will already have 180mm of extension (functionally) built into it. If you add all three extension tubes for 68mm more you will have 180 + 068 = 248mm of extension on a 180mm lens for a total magnification of 1.38X magnification.
If you put the same extension on your 90mm macro you have 1.75x magnification.
Here's a shot using 68mm extension on the Tamron 90mm (uncropped) -
and a 10% crop of that frame -
Need more magnification than that (really?)?
If you put the same extension on a 50mm 1:1 macro lens you 2.4x magnification. Of course, 2.4x magnification with your camera means you would fill the frame with something just over 9mm wide! You can zoom in to your shot beyond that to get just a single millimeter of subject in the frame.
Do you need more magnification than that?
If you want to go much beyond 2x magnification you are better off with something like the Canon MP-E or a set of auto-extension bellows. With a bellows you can get really high magnifications (10X+).
You can also get high magnifications by mounting a microscope objective directly to your camera, but that is more difficult to do and ends up being more expensive to do it well. There is a lot on line about this.
Of course, your subject distance would be very small with these shorter lenses.
So if I was you, I would spend $100 and buy the extension tubes. Play around with them with any of your lenses you have and see if they get you what you want. If not, go to the more expensive options.
Long-toed Salamander Courtship Ritual by Corey Raimond, on Flickr
