I think your question doesn't really correspond to what you're looking for. Bang for buck is in the smart phone, because whether or not you have a bike computer, you're likely to have a smart phone. So to get bike functionality, all you have to do is add an app, and that's either free or cheap. I use my iphone and ridewithgps. I pay $50/year for the service, and I could get away with the free version. I look at my distance and speed and my speed up hills. This helped me improve my speed, distance, and my hill climbing.

There are some people who are better (in speed, distance, climbing ability) than I am, and to reach their levels, perhaps the best thing for me is to get a bike computer with sensors. That would give me more accuracy. My app running on GPS alone isn't super accurate, but it's good enough for me.

If you want to climb better, try to stay out of the saddle for as long as possible. With each climb, try to stay out of the saddle longer than yesterday. And at the top of a tough climb, you'll be slower for a minute or two, but try to recover your breath and get back up to speed as fast as you can.

On long climbs, try to find a pace that you think you can maintain if, hypothetically, the hill went up for the rest of the day. Alternate in and out of the saddle.

And this will sound weird, but I say don't shift down too low. Once you shift down, you're not going to shift up before the climb is over.

Follow these techniques, and you may not be a world class climber, but you will get better. I managed to survive and finish the Hillier Than Thou this past September. It had a ton of very tough hills. The warning on the web page says, "If you don't train for this ride, you will regret riding it." I was not at all the fastest participant, but I did OK, and I was one of the older people, and I certainly had a heavier bike than everyone on the ride. So I was pleased.