I agree with the previous poster, Iron Horseman. As in so many instances, the up-front costs CAN be horse-choking, but it all depends on the parameters of the purchasing requirement that you set for yourself. If you have several/a lot of locomotives that you must have converted, either by yourself or others, then you'll have to include the costs of each locomotive's requisite decoder, plus any other oddities such as a resistor here or there, plus its installation if you don't/can't do it yourself. When you purchase your DCC system, you'll want to know what demands will be placed upon it, both in terms of how many decoders will draw amperage through it at any one time, and how much amperage that will amount to at a peak load. If you expect to have friends over to run several trains, you'll need extra throttles, but each of those throttles will control a decoder with an amperage throughput to a can motor...or two. The can motors are what take up about 90% of the amperage at any one time.
Your base controller, the brains of the system, if it must be new in box, will set you back at least $130 these days, and you can go up to $600 for some larger systems with duplex throttles (meaning radio, no coiled tethers to the throttles which limit their mobility around the train room), or much more than a mere $600 if you have to have one or more boosters or auto-reversers.
If you are unlikely to ever have more than three decoders running motive amperage to their slaved can motors at any one time, you'll never need more than about 3 amps. If controlled individually, you'll need three throttles (except in the case of the DT400 series made by Digitrax with their two encoders on the paddle, each a throttle). If the three locomotives will be consisted, as you'd see in MU'd diesels, then you'll only need the one throttle. You can see that you must have a firm idea of your requirements before you pay for even the one shipment...or you can add even more shipping costs for something you didn't know to order.