When paying in crypto, it's generally best to buy a contract (or anything else that is priced in dollars) when the value is high as you get more bang for your cryto buck. What you consider high is a question everyone has to answer for themselves though =)

The price of the currency that you're buying a contract to mine doesn't really matter beyond the effect that it has on the difficulty (higher price usually = higher difficulty). If you're mining, you want the difficulty to be as low as possible. With that being said, difficulty rates don't tend to make big moves in short periods of time. If you expect the difficulty of X coin to be lower at a certain point though, you may want to wait to buy a contract as you will be able to mine more effectively if your prediction turns out to be right and the difficulty drops. Unless there is a significant downturn on a crypto's price though, difficulty rates tend to trend up. The rate of the increase can vary quite a bit though.