You need a decoder for every locomotive. You don't need decoders for switches or power acessories unless you want to. Personally, I think controlling switches with a decoder is overkill. If you are using "walk around" control, and with dcc, any other system is probably not as satisfying, it is simple to install control buttons or switches at each turn out on the front of the layout nearest the switch you wish to control. Basic decoders that will control speed, direction, & lights are not very expensive. Generally less than $25.00 per decoder. If you are going to buy locomotives, but haven't bought all you will want/need yet, it is less expensive to get them dcc equipped than to install a decoder yourself. When you get into sound decoders, the price goes up considerably, but you don't need sound in every locomotive. A switch engine that runs alone most of the time, you might want sound in; but if your other engines are run in consists, you would want to consider just equiping one unit in each consist with sound. If you have an A-B-B-A set of f-units, put the sound in one B unit. If all of your locomotives have sound, and you are running a number of trains with multiple units in consist, and all are sound equipped, the noise could be a bit too much. You can get some basic starter sets for dcc operation for under $200.00, maybe even around $100.00. A good dc throttle/power pack will run you $25.00-$35.00. If you want to run more than one train, you will need a dc throttle for each train. When you get to three throttles, you are probably close to the cost of a dcc starter set. When you add in block control switches, you can easily spend more on a dc system than you would on a dcc system. Dcc systems get much more pricey when you add power to be able to run more trains with multiple units. I think the basic systems will run 3-5 amps, which is enough power to handle 6-7 typical ho scale locomotives with can motors. If the motors are open frame, the total is probably 4-5. When you go up to 10 amps of power to allow you to run 10-12 locomotives, and then you divide the layout into power districts with a second or even a third 10 amp power supply, then the costs will sky rocket, but the number of dc throttles required to run 10-12 trains would cost as much or more than the cost of a large dcc system.