Wiring the Aprilaire 600 without using the 24 volt transformer or the "extra" induction sensor relay:
NOTE: Whether or not the roughly $35 for the "recommended" induction sensor is worthwhile depends on the installation. You don't need it if you have a furnace with a "HUM" terminal on the furnace's PC board (most newer furnaces) that provides a 24 volt supply. The only way to know is to read the furnace documentation or check it with a volt meter.
If instead of a 24 volt supply you have a 115-120 volt supply, follow Aprilaire's instructions for installing the 24 volt transformer (included) and you'll have to pony-up for the induction sensor.
Here's a typical installation WITHOUT using the transformer or induction sensor.
Use 14-16 gage stranded wire (use 16 gage unless you have installation specific reasons for not doing so; available at Home Depot, Lowes, or virtually any ACE Hardware franchise).
The Humidistat CAN be mounted near your regular furnace thermostat if you're willing to run the extra wire or you can locate it as described in the installation instructions: At least 6 inches "upstream" from any other Aprilaire component. An additional suggestion I'd make is to mount ALL components downstream from the furnace filter if you have the choice.
Mount the main unit on the "return air" side and duct it to the "supply side" for a typical "Upflow" furnace. Despite what Aprilaire's instructions suggest, it's just too hot on the supply side. If you mount the main unit on the supply side, your installation may work but it will have a much shorter service life.
Connect the furnace board's "HUM" terminal directly to either 24 volt connection on the humidistat and the furnace's "Common" wire usually marked "C" to the other 24 volt humidistat connection. (The "HUM" terminal is typically like what you're used to seeing for automotive use: fog lights or nearly any other auxillary lights employ this connector type).
From the Humidistat, connect one of the "A/A" terminals to one of the Aprilaire (water valve) solinoid wires; then, connect the other "A/A" terminal to the other Aprilaire (water valve) solinoid wire.
The manufacturer suggests this is a two-hour project, and it could be if you have a service truck stocked with all the needed parts and an installation that lends itself to simplicity. The last 600 model I installed (which was my own) took a LONG day. You should probably plan to pick up parts one day and do the installation on a different day.
If you're comfortable making these types of electrical connections, you'll complete your project with savings typical of DIY vs. paying for a service call.
But, be warned: If you screw it up you can trash both the Aprilaire humidifier and your furnace, not to mention getting knocked on your rear or worse as the result of improper wiring.
If you're not comfortable making the electrical connections, do the mechnical work like mounting the humidifier, controller, running the duct, and plumbing-in the water supply then have an electrician handle the final install and test per Aprilaire's "checkout" instructions and you'll still realize a substantial savings.