You may check the differences in monitors and calibration tools there, lots of practical info.
iMac does NOT have the type of graphic card and monitor to allow you a big jump in color reliability and performance. Any calibrator will do only a software calibration which doesn't really change the monitor, just some software bits on the graphic card driver and in order to show you how efficient, how advanced and how much better your monitor looks now after the calibration they all generate a sample screen with some colored pictures and let you click on the AFTER button to check the effects ... No tricks there
This change ALWAYS looks the same no matter how many times in a row you calibrate your iMAC... you know how much it matters...
Besides, the monitors of the Macs are very pleasing to do computer work but are too much dead-end in terms of calibration, color accuracy and are over-contrasty too (deceptive psycho-trick to imply good photos- high contrast, high luminance, very bright... in the shop you can't look away.) And you can't really turn it down. In reality a printer cannot deliver that much contrast (less than 30% actually) and you can't really do proper color work with them, so => save your money. Find a hardware calibration monitor first and buy the calibrator for its system.
I use a 27 and a 30 " NEC PA both calibrated in the hardware of the monitor at 16 bit - this is not even touching the graphic card of the MAC PRO - by the Spyder3Elite. It does a good job there even if there are better options nowadays. But if I try to calibrate the 400$ 24" Samsung (which is actually a good system monitor, fast and contrasty, similar to the Samsungs in the iMacs or MacBook Pros from 2009-10) I just shake my head at the results and laugh when I see the BEFORE/AFTER screens
BTW, did you know how expensive photography can be when you bought your first DSLR...