So those voltages are only valid if the battery is not under load. At least wait a few minutes after you remove the load. The longer you wait, the more accurate the reading will be. It will be low at first, but over time will creep up. The first few minutes will produce the most noticable difference.
The digital voltmeters are pretty accurate. A simple voltmeter will measure the voltage accurately, but will not take load or temperature into account. You will need to compensate for that. I haven't looked into all of the really expensive meters, but to be accurate at all under load, they would need to measure charge current and load current over time to approximate the battery state of charge. And they aren't perfect, especially with just one battery. They may also require a temperature probe stuck to the battery.
It's just my opinion, but to me, if I had $300 to spend, I would get a solar panel and small solar charge controller before I got an expensive meter. It would be a lot more helpful over the long haul.
