Actually AA size batteries are more preferable than battery packs. You can get a charger & get 2-3 sets instead of a single spare pack which will cost you more. Again if you run out of spares, AA can be got from almost anywhere.
I have Alkaline rechargeable for my old cam...the alkaline cells were more powerful but charging them takes almost 10hrs..
The charger + 2 Alkaline cell costs nearly Rs.700-800
And if u get a better and faster charger u will spend 1k+ on charger alone
All these adds to the budget
You're right but its just a one time investment. I've 4*2700mah alkaline batteries which give enough power to shoot for 3-4 days with moderate flash use. if without flash they can go on & on for around a week or above.