$20 to $30 for a normal lenght LP hose is pretty reasonable. $35-$40 would be ok for a longer 5 to 7 ft hose.
As for lenght, the best indicator is your current hose and how well it works for you. If it feels like it is pulling your second stage to the right a bit, a hose 2 inches longer is in order. If it feels like it is pushing the second stage to the left, a hose 2 inches (or more) is in order.
Most primary regulators come from the factory with 22", 24" or 26" hoses. 20" hoses tend to be found on low pressure inflators while 30" and 36" hoses tend to be found on octopus second stages and 40" hoses are the most common size found on regs used with stage bottles and decompression bottles.
You may also want to consider a long hose in a 5' or 6' lenght. 5' works well for skinny to normal sized recreational divers while a 6' hose works well for larger sized recreational divers. The hose routes under the right arm across the chest and around the left side and back of your neck and generally does not push or pull the second stage. It makes air sharing an absolute breeze and in my opinion they will continue to become increasingly common in the future for recreatioanl divers. I have never shared air with a diver who was not impressed with how much more smoothly it works with a long hose.
A 7' hose is designed for more technical situations where the extra lenght may be needed for two divers to exit a wreck or restriction in tandem. The extra lenght needs to be routed under a light cannister or tucked under the waist strap. They do just as good a job for recreational divers as long as you have a means to keep the extra hose under control.