You can use
Entropedia's weapon chart. Type X5 in the 'Name' box, and you'll only see the Emik X5 listed. Now you can tick 'Include markup' (markup is about 280% there atm), you can also tick 'Custom values' and enter the markup you intend to buy/have bought the gun with.
Hope it helped a bit
Oh and another thing, you can also tick 'Skill based' if you have your skills uploaded to Entropedia, and it will show the gun's eco according to your skills.