Can you tell me what's the rough different bottom time between Air & Enriched Air?
That's a difficult question to answer directly, because it depends on a number of variables.
As a general statement, Nitrox will be most useful to you at depths deeper than about 60ft because for a lot of people that's about the depth where the NDL starts becoming the limiting factor in the dive instead of the amount of air in your tank.
Nitrox will give you longer NDL's but how much longer isn't a fixed amount. It depends on the depth.
I can give you a couple of examples (NB: don't use these numbers for planning your dives. or if you do at least verify them!).
Assuming you're diving on 32% instead of air (21%)
At 60ft the NDL for air is 55 minutes using the PADI RDP. On 32% it would be 80minutes.
At 100ft the NDL for air is 20 minutes. ON 32% it would be roughly 25 minutes using the table although in reality a computer would give a few more minutes (more like 30) because the table calculation includes a bigish rounding error at this depth
at 115ft, which is about the deepest you're supposed to dive using 32%, the NDL for air is 14 minutes but for EAN32 it would be 20 minutes.
Clearly, the shallower you are the more extra bottom time you get but unfortunately that's game of diminishing returns because at depths shallower than about 60ft you'll usually run out of air before you run out of NDL, so those longer NDL's don't help you at shallow depths.
I've already signed up for AOW but I'll call about Enriched Air as well if it's worth for that price.
I don't know if we can answer that for you. If you're making dives now that you need to shorten up because you're running out of NDL then you'd get some benefit from it. If your air consumption is still the limiting factor in your dives then you would be just as well off to delay it.
R..