iOS apps generally tend to sell in the $0.99 to $2.99 range, some successfully going up to $4.99 and more..but those are few and far between..
That being the case, the first thing to determine is what your final price of the application is going to be. If it's going to cost $20 or something, then having crowd-sourcing where people can pay you a smaller fraction to get access to the app is one thing, but if you plan on selling it for the usual iOS app pricing, then there's little advantage to crowd-sourcing, unless you get a LOT of people pledging pennies to you..
IMHO, crowd-sourcing your project shouldn't replace regular capital generation or investment on your part as an Indie developer. You might find that the regular marketing routes of create a website, providing some nice videos of your application and then handing out some promo codes to websites dedicated to reviewing iOS applications might garner enough interest to get your app moving along.