The very rough calculation is that you lose ~20% ee per tier. This means that if you have a 160% ee level 45 charm it will become the equivalent of a 140% ee when you hit 50 and a 120% ee when you're at 55. So it all depends on how big the difference is between them

And the reason that level 55 items with crappy stats cost more than level 45 items with decent stats is because people are more likely to mute level 55 items so the price on them is hella jacked up. Last night I got a white Ice XIV as a secondary drop from a well and I welled that and the stats were shit but I put it up on the MP for 50 and it was sold in under 15 minutes

And if you need any more help feel free to PM me