why is SMA cable so expensive?

Yes, cable has loss. The higher the frequency, the greater the loss. Also, the smaller the cable, the greater the loss at all frequencies.

As to why it costs more than say, RG-6. Well, they make RG-6 in the millions of feed yearly. The small cable is not used for long distances so it is typically used for short length applications. So, the manufacturing costs are higher per foot. Add in the gold plaited SMA connectors, you get a much higher cost for the cables. Simple.

Isn't that called "line drop" and don't we have "decibels" because of it?

Why does anyone pay $100 for an HDMI cable? I get mine for 6 bucks each from the Same Factory In China (tm).

If you can get manufacture's cost, great. If not, that would double for the Wholesaler, then double again for a retailer. (Rule of thumb)

As to why we have dBs. That would be because of Bell labs looking for a way to simplify losses and gains. Loss and gains are dividers and multipliers. Using the log base 10 table and renaming it the Bell table, was the simple way to simply subtract and add to explain the system. The direct use of the log table was a bit too cumbersome though because it involved a lot of decimals. So, someone thought it would be great to multiply that by 10. (Hence, deci - Bells.)

Isn't engineering fun?

Yeah. The original "bel" was basically about "line lag" as they strung the telephone cables down the highway.