Several factors affect what it costs.
The main factor is whether you are relocating to Greece for the first time and you apply for exemption from any taxes/fees associated with bringing your own property. To do that, before
you relocate you must complete paperwork to prove that you are relocating, such as tax forms, utility bills etc, from your country of residence as well as paperwork related to the vehicle. Details are on the UK Greek Consulate web site
. In that case there should be very low administrative fees. This all needs to be done before you relocate to Greece so I doubt if this route would be open to someone who has already moved here several months ago and brought a vehicle with them.
If you don't satisfy the exemption condition then you have to pay a bigger re-registration fee that depends on the cost of the car new today, a depreciation factor depending on its age and, more importantly, the engine size and what EC Emission Directive it meets. The fee is a percentage of the depreciated cost and the percentage goes up with both engine size and level of pollution. That is why one person says it is quite reasonable, perhaps 7 % and another says it is astronomical, say 385 %. I have a booklet as a series of jpgs; if you want it PM me your email address. It's a bit old, 2001, so you might be able to get a more up to date version.
PS If you use the exemption route you should get red plates which restrict what you can do with the car for a few years (5?). If you pay the fee you should get normal plates and it is the same as having a car bought in Greece.