Depending on what kind of riding you usually do, you can install REAL cruise control for around $100. Do some searching around the Internet and you'll find that many people have installed the AudioVox CCS-100 cruise control on their bikes. This is a
real cruise control unit that is made for cars but can easily be adapted to a bike. It has a small convenient control pad that can be waterproofed and mounted easily. I have mine mounted on the handlbars next to my clutch on both of my bikes:
I have it installed on both of my bikes and will never have another bike without it. If you can't find a reference to where someone has installed it on your bike, it's really not that hard to figure out if you have some basic electical skills and know how to install 12V electronics. Just read up on how other people have installed it and that should guide you.
I have ridden with both throttle lock devices and the real cruise control and find that it dpeends on what kind of riding you do whether a real cruise control or throttle lock device is best for you. If most of your riding is in heavy traffic where you constantlly need to make minor adjustments to speed, then a throttle lock device is better as you can just back off on the throttle to slow down a bit.
If most of your riding is on more open roads, then real cruise control is best. It allows you to just set your speed and forget about it.