$20 for a loco or a multiple unit.
$40 for a route
$42-48 for a route twin pack
Let's for say that the route or loco you are interested on trying but then you think that the price is wrong. You can wait for Steam sales but it will take months just to buy DLC in a discounted price. Some DLC are old and deserved to be bought during a sale. So many locos and routes that I had driven are buggy and may need some fixing. Dovetail does a massive job trying to get some DLC updated.
The game has grown to over $5,000 on DLC and who would have the money and buy them all at one time if you wanted everything? Dovetail wanted to have a selection of routes that people wanted to choose from. I do like the game but if Dovetail wasn't considering overpricing DLC and include them in the game then a lot more people would enjoy playing the game more than having to start with a few routes and several locos at their disposal.
As for me, I almost own all the DLC and I always say buying the DLC in route/loco twin packs, collection bundles. At least you can save money that way. Every year they will update the core version of the game so if you want to buy it then you can. Those who reviewed Train Simulator 2016, Dovetail Games has to afford the payments that the licensing that they use, pay their employees, and their production.