In general, all smartphone and tablet batteries will last about two years before needing to be replaced. This applies regardless of mAh rating as it's a limitation resulting from the materials that make up a battery.
You would think that, all things being equal, device X would be equal in price to device Y. However, it doesn't work that way. As a company, an OEM is in the business of earning money. OEMs want to recoup the amount of money they spent in not only doing research and design but also manufacture and marketing. As a result, the OEM will price the device in such a way as to recoup those costs based upon the quantities sold and make some profit on it as well. Thus, since things are decidedly unequal, device X costs more than device Y despite having nearly identical specifications because of differences in the amount of money spent in making the product or advertising it.
The pricing above is the wholesale cost. For most devices, this is the price a cellular carrier will pay for the device. From there, carriers have to do some research and design into ensuring the new device is stable on their network. This sometimes requires modifications to the firmware. Research and design costs money, and the carrier passes these costs onto their customers. The carrier though is also in the business of earning money. Therefore the carrier prices the device so they can both recoup their research and design costs and earn some profit from the device as well. So the gap widens between device X and device Y.
The price you pay for a device depends on these factors.
I have Samsung Tab 2 which is pretty old right now and the battery seems pretty good because i rarely use it. You are right, every battery life is really depended on the condition and how long we use it.