You’ll know this only from testing. It’s different for every product, market, service, and offer. It depends on how much your product costs and how much you spend on your ads. Spend a few bucks, and then pause and assess the results. It’s impossible to give accurate figures for CTR and purchases unless you compare against someone in the same vertical.
Expect to see lower CTR and purchases from Facebook than from a search engine ad campaign. When people surf Facebook, they are not intending to buy something, even if they click on your ad. Mostly they want to see what is new with their friends.
If someone searches for a term related to your product or service on Google or Bing, then she has intent and is more likely to click on the ad and then purchase the product.
So rather than focus on expected purchase rate, just focus on return on investment. As long as you have a positive ROI, keep running your ads and landing page, and ramp up your spending while continuously testing variations to attempt to improve CTR and purchases. If you have a negative ROI, make changes quickly or abandon the campaign and move on to another product or media venue.