For an inexpensive panel meter check out ,
http://www.allelectronics.com/make-a...s-Panel/1.html
To do an actual "STATE OF CHARGE" test you need to use a hydrometer (SP?) , or in other words , take a specific gravity reading on each cell relative to ambient tempture and do the math.
As far as I know there is no-one that makes an "affordable" panel meter to do this.