A former rule of thumb is that they wanted a reasonable chance at getting 10x their money back in return after a few years. Why? Try these crude but reasonably accurate statistics:
* 70% of the companies they invest in will go bankrupt, losing all of thier money.
* 20% of the companies they invest in will go sideways, just barely paying back their money.
* 10% of the companies they invest in will boom, returning roughly 10x their money.
So, imagine you invested $1M each into 10 companies. On seven of them, you'll lose all $1M. On two of them, you'd get your $1M back. On one of them, you'll get $10M back. Total return: $12M on a $10M investment. That is just like earning 10% interest for two years. They could do that just by buying mutual funds, so why invest in a start up? If they don't see a way to get at LEAST 10x their money back, they won't invest.
The new rule is more like 30x. (with the stock market crash, they aren't as free with their money). So, you want $50,000. You have to show a way that in a couple of years that their portion of your business will be worth $1.5 million (30 times the initial investment).
Now to answer your question. Suppose you had a business model that showed your business will be worth $2M after a few of years. In that case, they'd need $1.5M / $2.0 M = 75% of your company to make that investment. Suppose on the other hand, your business model predicts that your company will be worth $10M after a few years. In that case, you'll need to sell $1.5M / $10M = 15% of the company.
To answer you, it all depends on your business predictions. And they have to be reasonable, realistic predictions. Telling people that an online pet store is worth billions of dollars doesn't work any more.
Anyways, $50k is way to small for venture capitalists. They'd laugh at you. They invest $1M or more generally.