Hi Lost,
Yes you’ll find tons of ready made lehnga shops in Islamabad, very good ones. She can even go to markets in Rawalpindi which is right next to Islamabad as Rawalpindi is much cheaper and you can get the same stuff. There are tons of shops.
Average price completely depends on where she buys it from. But, in my opinion, you can get a very decent nice lehenga with the best silk material tailored and choli full of kaam for within Rs.5000. (i think the exchange rate these days is like 1 us dollar = 60 rupees, u can get the exact rate off the internet 
)
You can even be innovative with the material and the kaam and get a very nice looking lehnga done for as low as Rs. 2000, but then the material won’t be the best silk and the kaam (embroidery) might not be so filled.
I have never gotten a ready made one but the good quality ready made stuf is more expensive, I’d think about 6000-8000 from a normal shop and even like 20,000-30,000 from an upscale boutique, or even more, but that is a complete waste of money. Ready made stuff in Rawalpindi will be much cheaper than in Islamabad. I think though you should just send her your measurements and tell her about your taste and ask her to get it taillored, thats so much more economical.
If you do decide to go for the ready made one though, then if you are still in touch with her then just send her your measurements as well so she canl get the top altered by the darzi before she brings it back here because someone here might spoil it and over there the tailors are used to doing this kind of stuff.
BTW, ask her to get you matching choorian (bangles) and matching artifical jewelry (atleast a heavy neckpiece and earrings set, and paanzeb (anklets), challay (rings) and if you like tikkas, the forehead jewelry u know what i’m referring to? ) with the lehnga as well, as well as a small matching purse to go with the outfit. (if she is getting the lehnga tailored then she can get the purse made from the same material). all that together will look sooooooooo stunning 
All in all, I think if you give her a relaxed budget of Rs 12,000, she can get you the very best stuff and if you want good quality artificial jewelry and accessories with it too which i think you should really really get as without that half the beauty of the outfit is gone! so that much should be enough…unless she decides to get the stuff from places that grossly overprice everything…
Feel free to PM me if you have more questions 
Best of luck 
and enjoy the lehnga