if you expect a 6* from step ups you should plan to buy 2-4 of them at the step 7 level. Getting anything before spending 300$ is extremely lucky.
Your money will be much better spend if you plan for an event and buy packs when you need just a little help to rank or get a reward 6*.
This is also the 2 or 3 topic addressing the exact same question. You should search for similar topics or put your quick question in the FAQ sticky.