Really depends on the title. DTS discs are rare except for titles they made a bunch of. They made a hell of a lot more copies of the AC3 discs.
Basically supply and demand.
And you can wait for a deal to come along every so often. Some titles are never going to come down in price due to their scarcity though. Though i found DTS discs of Goldeneye and Tomorrow never dies for 40 each and i find that very reasonable. What isn’t reasonable is like a hundred bucks for Mortal Kombat if one shows up for sale.