Can I buy a medel in 5,000,000$?

Because the metal style says thet it costs 5,000,000-20,000,000.

That is the range of cost for that material.

You can buy the cheapest medal for $5,000,000, yes

So the differnce are in the look?

like the 5mil is the most less pretty and the 20mil is the most pretty?

Well, possibly.

The point is, the more expensive ones are the least common, and therefore, the most honourable to be awarded, because it took more effort by your guild.