Know-How Exchange : Advertising/PR

There's no shortcut to doing this work yourself.
Contact local websites to find out how much ads would cost.
Determine how much for Google PPC ads based on your local keywords.
Do you have a website? If not, you need one - and likely SEO to help improve your organic search results.
Where is your competition advertising online? How much does it cost to advertise in these same locations?

Why do your ads need to be on other people's websites? Why not simply use search-based PPC, Facebook, and Twitter advertising?

A business plan within 48 hours? Who is holding that gun to your head? A really good business plan needs some "incubation time," so you can poke holes in it and then plug them.

You need to do the research, as Jay Hamilton-Roth suggests. My knee-jerk reaction is much like Gary Bloomer's -- PPC and Facebook advertising. But it all depends on your objective and resources -- time and money.

If this is more than you can handle, then you'll need to either adjust your objective, or find investors/consultants/partners who will share the load ... and receive an attractive return on their investment. If you can't promise them that, then you probably shouldn't be doing this yourself.

And if this is all for a business development program (and not for addressing a real need), just make up stuff to demonstrate your understanding of the process. You won't be able to do much more than that in what's left of 48 hours.