A lot of phones seem to drop in price about 3-4 months after release, but at the same time the carriers tend to order less of them after the initial launch, so once supplies run out, they are usually hard to come by again.

But you do bring up a good point with regards to new release phones. With the prices of smartphones getting relatively out of hand and pushing the $2k price barrier in the past few years, the smart thing to do would be to buy phones that are one generation behind. When the S10 comes out, the S9 would offer the best bang for your buck. Same when the iPhone XS came out, the better deal is the previous year's iPhone X. If enough people hold off of the new releases, eventually the phone manufacturers and carriers will smarten up and make reasonably priced phones again.