Closeup filters are cheap. Should be less then $20. I bought a 58mm Hoya +4 recently for my fren for $13. If you want to get a macro filter (eg. Hoya +10) then I think it is more ex... i think $60+ but not too sure. But generally +10 is too much for insects as you will get too close to the subject, and a +4 is sufficient.
Briefly....two achromatic elements to correct chromatic aberrations.
In short, means lesser loss in quality. logically the more glass you add to the front, the higher the loss in quality. 2-element ones reduces this and almost lossless. Compare normal Hoya ones and a 2-element like Nikon of Canon and you will clearly the image quality difference.
Once a closeup filter (diopter) is attached on to the lens, the focusing distance (at the lens infinity focus) will be determined by the diopter, regardless of the lens.
Hence, a +2.9 will have a focusing distance of 1000mm/2.9 = 334mm, a +1.5 will have a focusing distance of 1000mm/1.5 = 667mm. A +10, is then only 100mm....too close for most live subjects.
You can't use any diopter on any lens though. You have to make sure the focusing point is outside the lens, ie you obviously cannot use a +10 on a lens that is 30cm long, for example, coz the focusing point is inside the lens!
1) The focusing distance is measured from the focal point(dunno term rite onot...heh), always somewhere in the middle of the lens (hence inside), one of the glass element is used for this purpose.
For example if the diopter is +3, the focusing distance will be 33cm, but the focal point of the original lens is somewhere 13cm from the front thread, so the object have to be at 20cm from the front thread to be in focus.
Because of this, a macro/close-up lens works best with a zoom lens, since the focusing ring does not work anymore the fine tuning is done by the zoom ring.
2) the 62mm ones (4T, 6T) cost the same, 52mm thread (3T, 5T) i dont know how much (guess shld be 70+)
3) YES! quality losss is near zero, I rather use my 135mm 2.8 Soft Focus or 28-135 IS than a dedicated macro lens ( a bit too expensive for the purpose) but think can consider the Sigma 180 3.5 EX.... I think I will get 5T soon to add to the power....