With conversions, you're telling FB to send you traffic from people most likely to purchase. With link clicks, you're asking them to send you traffic without that intent, and just likely to click - which is usually lower quality, on of course cheaper. I've heard of it working for some people, but personally, link clicks has always been negative ROI for me. Regarding bidding, it's fine to overbid, especially while testing. Keep spending money, testing different things, and you'll figure it out.