The lowest price beach cart power kit with batteries that we have is $300.00. Here is the link to it: https://cart.electricscooterparts.com/ProductDetails.asp?ProductCode=KIT-145-C
This kit only includes a rear wheel sprocket with no mounting hardware, so a way to mount it to the wheel would need to be figured out and followed through with. This would only provide one wheel drive though which would work good on flat paved and dirt roads and trails however one wheel drive would not work very well in deep sand or uphill dirt trails or roads.
For good traction in deep sand and uphill dirt roads and trails two wheel drive would be needed which would require replacing the existing rear wheels and axle with a live axle and wheels that attach to it such as can be found at go kart parts stores.
Please let us know if you have any questions.