Always ALWAYS ALWAYS Read the Contest Rules (And Then Back Away Quickly)
For example, CUSTOM BOOK Publications.
The stock art and grammar mistakes on the front page should warn you. The contest page isn't much better and just looks scammy.
Average limit of entries? What?
And the prizes are... well, the same kind of vague language that tends to mask scams everywhere:
...guaranteed traditional Publishing Contract with Custom Book Publications in both print and eBook formats, a professional edit contract, personal webpage, and marketing and distribution of your published book and ebook.We don't know what the terms of those contracts are, mind you. Just that you'll get one. And no guarantees about the quality of marketing and distribution or web page. Oh, and $300 if you're the winner.
In addition, the winner in each category will receive a cash prize of $US300.
What's it cost? Well, on that page it mentions $39 as an entry fee. But wait, there's MORE... Look at the grant of rights on the rules page:
By submitting an Entry and until formally rejected or eliminated from the Contest, you grant Custom Books the exclusive first publication rights to your Entry in all formats. Whether a winner or runner-up or not, if Custom Books wishes to publish your Entry, you agree to negotiate the terms and conditions of a publishing agreement exclusively with Custom Books for a period of 60 days after you receive notification. If you and Custom Books have not reached agreement such period, you may offer the work to other publishers on the condition that before you enter into an agreement with another publisher, you will afford Custom Books the last right to publish your Entry on the same terms and conditions offered by any other publisher, plus an advance against royalties 10% greater than the other offer. Custom Books must communicate its decision to you within 5 business days after you have provided written notice together with a copy of the full and complete offer on the other publisher's letterhead as evidence of the other publisher’s terms and conditions.
TL;DR: They do their damnest to grab the rights to your book. Oh, and they can publish parts of it on their webpage.
Tell me how that's useful to anyone's career as an author.