There are different rules people follow. My personal rule is that I won't buy into anything with more than 5% of my BR. So $5 would be enough for my to play Bumblebee SNGs on Carbon and nothing else. Those have a very high level of variance so I didn't play those much anyways when my BR was so small. I personally include the tournament fee as part of the 5%. So for the .25+.5 tournament I would need $6. There really isn't a very good selection of stuff to play until I could afford $1 entries which meant my BR needed to be $20+. I follow this no matter how small my BR is. I started from freerolls, so that meant the first few weeks were basically FRs only until I had built up enough to play in anything else.