How many emails do spammers have to send to get a sale?
Here's a resourceful attempt to find out...
http://www.icir.org/christian/spamalytics/

Spam conversion rates
Various figures are quoted for this ( 0.00001% , 0.00036% etc), but it will vary depending on the thing being sold. Safe to say it's "a lot"! The kind of numbers you would need to send to make it worthwhile are likely to get you outed as a spammer, so if you fancy going for it anyway you may need "Online Reputation Management" services.