Doesn't make any sense to just start throwing money at it without knowing what the real problem is. Quick way to test the solenoid is take a screwdriver that has a PLASTIC HANDLE, turn the ignition to on, and short accross the two big copper looking studs on the solenoid. If it cranks over just fine or cranks and catches, then you know it's the solenoid. If it does the same thing where it just clicks and nothing then you're starter may be dying or does need a heat shield.

A hot starter takes more juice to crank over, so you may want to also take a look at your battery cables and the grounding point of the negative cable. If they're corroded that could also be a problem as the corrosion adds resistance to the current.