Interesting rule of thumb based on military operational research performed by Frederick Lanchester during World War I:

If you’re trying to enter an existing market where the dominant player has greater than 40% market share, you will need to spend at least 3x the sales/marketing budget of that leader to succeed in a head-on attack.

However, if the largest player has less than 25% market share, all you need is 2x the sales/marketing budget of the leader to succeed – the cost of entry is much lower.

Thus, if you're a startup confronting a formidable incumbent that you lack the financial wherewithal for a frontal assault, consider re-segmenting the market to create a submarket where your product can be unique or substantially different. As an agile but (usually) dirt-poor startup, it’s crucial to pick your battles carefully.