If I was you..
Don't let them know you're just starting out, tell them you been making tracks for a while now, that you fucked with alot of people they probably never heard of. Get a solid rate and package rates in your head before talking money. Start high. 9 out of 10 times they'll try and talk you down anyways. Offering exposure, credits. etc.. is cool, but if these (known) rappers are halfway serious, than I guarantee they're paying for tracks somewhere. If they're considering putting your shit on an album with other producers who they pay for , then don't think of yourself any less then them. Obviously you're hot enough for consideration.
if it all comes down to them not wanting to pay you, then the tracks probably aren't that great for them to want to break bread on. We all been through it, sometimes you take what you can get, other times you gotta check people right away or they'll take advantage of you forever.
A L S O _ NEVER GIVE THEM DATA BEFORE YOU GET PAID! AND DON'T LET FOOLS KEEP YOUR BEAT DISC UNLESS YOU TRUST THEM OR DON'T CARE ABOUT GETTING YOUR BEATS JACKED, OR USED ON MIXTAPES, ALBUMS, COMPS etc..
That kinda shit is gettin way too common right now, people don't give a fuck about anyone anymore. I got a track on C-Bo's and Messy Marvs last albums that I didn't even know about, came from beat discs that got passed around and looped. I'm out money and credits.