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OK, I see. The problem with flight time is that batteries add weight, so adding a larger battery is diminishing returns until it doesn't help at all. And battery energy density (of the best lithium-ion batteries) hasn't improved much at all over the last 10 years. In this way, it can be difficult to get anything that stays in the air for more than around an hour, and half an hour when you're carrying extra weight.
If you are software, embedded systems esp., a lot of the early quadcopter work was done with arduino setups. The nice thing about this is it makes the servo and motor control very easy, if you are designing something from scratch and have that kind of experience to customize control firmware, that might be a good starting point.
If you have a chance, to go the UAVSI show,
http://www.auvsishow.org/auvsi2015/public/enter.aspx
this will give you an idea what is out there, what other people are doing, and how much things cost.
BTW I the link to the DJI product you posted, I actually saw one of those at a local hobby shop just the other day. Nice setup, seemed lightly used, being sold for roughly $5000. (can't remember the exact price, and I don't think it included the gimbal). So I think that is the general price range for starting out with one of those things.
Used with a 6S 15000mAh battery, it can fly for up to 15 minutes.*
*Maximum flight time is tested on a windless day with a payload of 9.5kg, hovering at a height of 2 meters.
First, that battery alone, probably weighs a little more than 2 kg. And hovering a 2 meters, it probably hasn't fully left ground effect, and hence gets some benefit from that. How realistic a windless day is (or isn't) depends on where you live. But if you want to do a quick sanity check, let's say that payload of 9.5kg was mostly dedicated to additional batteries. With an additional 3 batteries of that type (~7kg), you might be able to make something that stays up an hour (if all you do is hover stationary at a low altitude), but that only leaves you with about 3.5kg for everything else. And once you start to maneuver around, your flight time will likely drop to around 30 minutes.
Now take a look at the same setup, without all that extra weight. If it draws 1000mAH per minute of flight when weighing a total of roughly 16kg, dropping the weight to only a single 6S 15000mAH battery and nothing else would put you around 6.5kg, so presumably it would only draw around 400mAH per minute of flight (if the relationship is purely linear) rather than 1000mAH. Add the same 3.5kg and you're up to 10kg, but that would likely still give you around 23 minutes hovering at a low altitude and that hopefully illustrates the diminishing returns you get when additional batteries.
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