Oh hey, I should actually answer the OP's question... so the antenna cost is all over the place, depending on antenna style. You can get an AS-3900 whip antenna system for around $150 a side, less if you find a good deal. That's the mounting bracket, spring base, and both sections of the whip. You can get the VRC-12-era antenna systems for a little less. All of these are going to have the same theft worries with the whip antenna just being able to be unscrewed (well, you could just always have a transmitter going to keep the antenna energized to be a shock hazard...).
Another thing that exists is the COM231 antenna, seen here:
https://www.cobham.com/media/99324/COM231 Final.pdf This antenna is a hollow mast, with the spring base permanently affixed. It's pretty unlikely that someone is going to go up with tools and unbolt your antenna mast. I know I saw beltfeld34 have one of these listed for sale, but I think he did want a bit too much for it. I personally paid less than $40 for mine in far better condition (and came with the tripod assembly from an AT-1011), but mine was an exceptional deal that probably won't be seen again. $150-180 is probably a fair offer for one of these, as they're rather rare, but do nothing more than an AS-3900. The COM231 was more of a prototype for later multi-band antennas with the same physical profile, which
would be worth a
lot more.