At the higher end I don't think the differences are too significant in terms of budget. At the low end digital can be significantly cheaper, although I haven't priced out film costs recently...
Just a quick look at B&H prices puts 100' of 16mm Kodak color negative at ~$50. That's just under 3 minutes worth of footage - you'll probably end up with 2.5 minutes per roll of useable footage. For a 90 minute feature that's a minimum of $1800 if you shot a 1:1 ratio, which is pretty unrealistic. More realistically you'll probably need to shoot a 3:1 ratio (likely even more) so figure at least ~$5k in stock, although you might be able to get it a little cheaper if you buy larger rolls.
So now you need to process around 10,000 feet of film - cinelab's rate is .20/foot with telecine processing, so that's another $2k. HD Telecine will add another $2.2k to the price.
So you're probably looking at around $8 - 10k minimum for stock and processing shooting a 90 minute feature at a 3:1 ratio in 16mm - and you still need to rent a camera & lenses for the shoot.
On the digital side you could buy a blackmagic cinema camera for $2k (or even a pocket cam for $1k), add batteries and storage for another $1k or less, and then rent or buy lenses as needed. You'll have no real limitation on your shooting ratios, faster handling/changing of media, the ability to preview & analyze your lighting directly in real time, playback on set, etc - all things that can allow you to work faster and more efficiently, potentially reducing crew costs as well. So unless there's some really compelling reason to shoot film it probably doesn't make sense from a budgetary standpoint - you'd be far better off putting the difference into things like lighting.