A contract will trade between 1 and 99 cents. This correlates to the odds that traders are giving to a particular event. If it happens, it resolves to $1. If it doesn't happen it resoles to $0. Let's say Biden is trading at 65 cents to win the election. So you can buy "YES" shares for 65 cents. That means there's somebody on the other side buying "NO" shares for 35 cents. Ok, now $1 is locked up. If Biden wins, the holder of YES shares gets the $1. If Biden does't win, the holder of the NO shares gets the $1. Simple enough.
Rule #1 of PredictIt: Stop buying YES shares. You can almost always buy NO shares of the other guy for cheaper and it's the same exact thing. For instance, right now Biden is trading at 66 cents and Trump is trading at 39 cents. Because people seem to put less than 5 minutes into researching how the site works, Biden betters tend to just buy Biden for 66 cents and Trump betters tend to buy Trump for 39 cents. Both of these people are making mistakes. If you want to bet on Trump you should buy "NO" Biden shares for 34 cents. If you want to bet on Biden you should buy "NO" Trump shares for 61 cents. In both cases you're saving 5 cents to get the same exact thing (actually slightly better because you get the field as well in case Biden/Trump die or drop out). There's an $850 cap, so if you bought $850 worth of NO shares, then you can go and start buying $850 worth of YES shares if you think its a good value. But always max out your NO shares when it's cheaper to do so (which it almost always is).
Furthermore, stop just putting in your bids at whatever the ask is. Look at the order book and then move your cursor over and lower your bid by a penny or whatever it is and then place your order. The market is going to wiggle around a little, stop overpaying for no reason. The exceptions are if it looks like the price is about to move up. If you've bought stocks before, this is like putting in a limit order instead of doing a market order.
I'll also quickly explain how buying "NO" shares of multiple people in a linked market works since it seems to confuse the hell out of people at first. I had to explain this to OSA about five times before he understood. Also, the numbers are going to be slightly off because of rake. I'm going to ignore rake for purposes of this example because it'll just confuse newbies more.
Candidate A is trading at 65 cents
Candidate B is trading at 30 cents
Candidate C is trading at 10 cents
You want to buy candidate A, so what should you do? Don't buy candidate A for 65 cents like a moop. Buy "NO" shares of candidate B. This will cost you 70 cents. Now go buy "NO" shares of candidate C. What happens when you buy the NO shares of candidate C? You might expect 90 more cents to be locked up, but actually that's not what happens because the market is linked. What happens in you get 10 cents released to you. So now you have 60 cents locked up and you have a NO share of B and a NO share of C. This means if B OR C wins, you will lose your 60 cents. You're essentially creating a parlay. That's why you'll see me buying NO shares of a guy who's trading for a penny. Even though the guy buying is getting 99 to 1, I'm not actually putting up 99 cents to his one penny. I'm actually receiving a penny for every NO share I buy.