I see that most people have the habit of not including shipping charges (plus VAT, when due).
On low cost items these charges are likely to be an important part of the total expense, in case of online purchase.
I keep an eye on macro prices lately, and I have seen all the lenses mentioned in this thread go on sale at prices close to the very low limit suggested by the OP. This happens ON OCCASION, because of errors in the listing, auction open to a single country, particular time of the year, etc, etc. Patience pays... but you still have to take into account shipping
The a Cosina macro, especially the MF version, can be had for little more than the proposed price (more so if it's without the original achromat dioptric needed to reach 1:1). The old f/2.5 Tamron Adaptall is often under 100 euros, and the Pentax (K or M) is more or less in the same range, but with more price variance. Once I saw an "experienced" Pentax-M sold for as low as 56 pounds plus shipping.
As suggested, a cheap way to get into macro is to use a diopter "filter" with a lens that already reaches close focusing, like a fifty. Diopters are not all the same, though. Cheap ones can be really terrible!
A good achromatic diopter with MC anti-reflection can be expensive, so unless you find a second-hand one for cheap, a macro lens would be a more sensible option.
Cheers
Paolo