I did a little research but blogs are never enough. I would like expert advice or even sample contracts which spell out everything if possible.
This is why you should have an agent.
Because the answer will depend on your country, what you wrote, the publisher, the current market, and so on. It is the job of the agent to know this stuff and negotiate the best contract for you.
Your current best path, as I see it, is to ask them honest questions, and hope that they will be honest in their answers. Which they very likely will be, because most publishers aim for a long term relationship with their authors and don't want to undermine that by ripping them off.
I wouldn't try to poker with what will be your long term business partner, that is not a good basis for a healthy relationship.
Some further words after the comments:
In my answer I presuppose that you know who the publisher (or agent) is.
If you go to a bookstore and look at the shelf with the kind of book you write, your publisher should be represented there. If what you write is not sold through bookstores, then their books should be (well) reviewed by whatever platform that type of writing has. If you look at the website of an agent or agency, that agent or agency should represent one or more of the relevant authors of your genre or field. If it is a small publisher or a new agent, you might want to contact their authors and ask what their experience was.
I really thought this one was a no-brainer. You want your book to be published by one of the major publishers in your section. Those are honest people. You don't need to double-check Penguin. If you are being approached by no-name publishers, then you don't only need to make sure they don't want to rip you off – you need to consider if they can even sell your book in the first place!
So, if you have written a book in, say, children's literature, I assume that you have widely read books for children, that you know who the other authors that write for children are, and who their agents and pubishers are. I assume that if you are approached by someone who is not on that mental list, you wouldn't consider publishing with them anyway, at least not without researching them carefully and making sure that they can even benefit your book.
Before you sign anything, read Kristine Kathryn Rusch's series on "Dealbreakers" -- clauses which, if they are in your contract, should make you walk away from the deal. For example, here's one about agents and the contracts you make with them. She doesn't seem to have a handy link where you can read the entire series, but this link comes close:
She has the term "Dealbreakers" or "Contracts" in the headline of all the relevant articles, so you can scroll down and find all the articles in her Dealbreakers series that way. (You might have to click on "Older Entries" a few times to get all the articles -- and the Older Entries link is dark-grey-on-light-grey, so it's hard to see.)
Always keep in mind, whether you're dealing with a publisher or an agent, that this is a business negotiation. Which means that even if both parties are being completely honest (which is true for some publishers, but not for all -- caveat scriptor), they will not open with the best offer they're willing to accept. Negotiate every clause in the contract, even the ones you don't think are going to be negotiable. You might be surprised.
And make sure you know ahead of time what your own dealbreakers are. You may not have ever thought about it before, so that's why Rusch's series is a good one. She quotes clauses from actual contracts, and explains in detail why they sound good in theory but are terrible in practice.
Also, since you asked about sample contracts, I'll link one more article. This is about a real contract that came up in the news recently because the publisher is suing the author for breach of contract. The article will do a better job of explaining it than I could, and will link to the actual contract itself:
Hope this helps. Good luck with your search for a publisher, and remember that one totally valid tactic in negotiation is to walk away and say, "These terms are unacceptable; I'm going to go find another publisher." Because ten years ago, if you wanted your book published, you didn't have much choice. But now, self-publishing through Amazon is always an option, which means you always have a fallback position at the negotiating table. And that gives you a lot more power, and will let you end up with a lot better contracts, than was the case ten years ago. So always keep that option in mind.
Never had a paid for publishing offer but I think the negotiation is pretty straightforward and can be done ... and is best done ... without an Agent. If you think you have a best seller on your hands then you would want part of the sales...if this is your first time start "big"(15%) and aim small (no lower than 10%.) Also take a look at the publishing house...have they ever produced a best seller? Talk to another Publisher when you're not satisfied...and you will not be...to see if you can get a better producer.
Be sure they have a good editor on the payroll.
I would not agree to any follow on material unless you've already written it.
Always take any money offered up front...even if it's 500 bucks.