Most battery/RC users will have a hand held throttle from their favourite system and a battery charger for their favourite battery type. These of course will have to be acquired initially and these sunk costs are the primary reason why folks don't like to change systems once they have one installed even though in a sense each loco is self contained. Personally I use both RCS and Aristo RC systems of varying types depending on the application.
Now for costs. Aristo is the least expensive. For high amp draw situations, I like the 27 MHz trackside board removed from its case and installed in a loco. This receiver/motor controller costs about $120 from a discounter (I get mine in Canada from Art Knapp). The installation will involve some switches, jacks and noise suppression so figure another $10-15 for components. If sound is required an accesory board will be needed (cost $70 approx) though I would recommend using a higher end system if you wish to install sound. The Aristo 75 MHz mobile has had range problems due to poor aerial choices and inadequate motor noise suppression - our club members have a wide experience with them and have finally gotten reliable operation from them. See www.ovgrs.org
for lots of battery RC details.
The RCS system has a number of ergonomic and functional advances over the Aristo and would be my choice if sound were to be used. Cost of an RCS Elite system of appropriate amperage runs to about the same price as an Aristo system including the accesory board.
Sound is easily installed with RCS and controlled with RCS - the cost of Sierra runs to a bit under $200 and of Phoenix just over $200 - I have never used other sound systems.
Installation of all this stuff can be done commercially and Larry's $300 plus is certainly a figure which you could work with. I have had some custom installs done by Dave Goodson and I have done some of my own (guess which ones are better!)
Battery packs can vary in price depending on whether you wish to use NiMH or Li ion, whether you have a custom pack built od roll your own, the number of cells you wish to use, and on whether you use Double As or sub Cs. For the typical B'mann steam loco, I would use 14.4V or 12 cells double As NiMH. I have an Aristo diesel powered by 19.2V of sub Cs NiMH and am in the process of building a heavy duty power car containing a pair of Aristo Li ion packs. I suspect that you can obtain battery packs from a discount supplier for well under $50 for NiMH while the Li ion may be a bit more. Make sure you have appropriate charging technology as the use of the wrong charger (especially with Li ion) can cause fires.
I have no experience with other systems though there are folks who swear by their Airwire RC.
I would also suggest that you take a look at the newly announced but not yet fully available GWire system that uses QSI sound - this may give a lower overall price for a fully configured system. Keep in mind that it is not yet available widely and that it may have some teething problems.
Similarly, Aristo has announced a new RC system coming soon as a replacement for their 75MHz mobile but it is also not yet available. Typical of Aristo products, I would expect it to be a low price leader but I would also expect a few idiosynchrosies along the way.
Regards ... Doug