How do I split revenue from IP with different owners?

It's actually the same answer regardless of what type of IP you're looking at - you have a contract and you follow what the contract says. Now, how do you come up with the system the contact spells out? Negotiation. Long, hopefully pleasant, negotiation.

How much does a patent cost?

12-15K from the time of disclosure to the time of filing. Then another $12-15K to argue with the USPTO. There are also some costs the office charges which I can’t predict because they change them (usually in October). I can say they don’t tend to go down. After that there are maintenance fees at 3.5, 7.5, and 11.5 years in. Those are also set by the USPTO. International patents cost more because 1) you need to pay for translation and 2) different countries have different fee structures.

What does the IP clause in my employment contract mean?

There are usually 2 or 3 parts to the typical “IP clause” you might find in your employment agreement. These relate separately to (A) inventions and patents, (B) trade secrets and confi-dential information, and (C) copyrights.
As to inventions and patents — in general, inventions belong at first instance to the person who made the invention, and (amazingly, to some people) NOT to their employer. Employers naturally want to lock down those rights, so they will require you to assign those rights to your employer.
As to trade secrets and confidentiality — one essential element to trade secret rights is that the material is actually secret. In general, employers will want you to promise to maintain the confi-dentiality of their trade secrets and confidential information. You will see this as part of a non-disclosure agreement or clause, often called an NDA.
As to copyrights — In some cases the employer will obtain the copyright in what you create for your employer just due to your employment. When this is not the case, the employer will require you to assign those rights to them. In some cases, the employment agreement will recite that what you create is a “work made for hire”, a legal copyright term

Who gets to decide if an idea is patentable?

Each country has a government office which evaluates whether your patent application should be granted. In the US, this is the USPTO (United States Patent and Trademark Office). The USPTO assigns a Patent Examiner to read and evaluate your patent application. You have an opportunity to argue or negotiate with the Examiner, you can appeal internally within the USPTO, and you can ultimately appeal to the Courts. The Courts can also declare an issued patent invalid if the invention is found to be obvious or otherwise unprotectable by law.

How do I get a patent as cheaply as possible?

If all you care about is cost, you can write the application and negotiate with the USPTO yourself. But this will take time and effort, and you are likely better served by asking a professional (a patent lawyer) who has experience with best practices for working with the USPTO.

How do I present an idea without a patent and still protect myself?

In general, without a patent, you will be unable to prevent people from copying a product you manufacture and sell, or a service you provide to the public. While the product or service is being developed, you might be able to protect the idea that gave rise to it using trade secret protection. For example, when attempting to look for investors or when engaging with developers, you can request that people sign a nondisclosure agreement (NDA) by which they promise to keep your new idea a secret.

How am I going to enforce my patent?

In general, it is not easy and it is quite expensive to enforce a patent. If someone is infringing your patent, you will have to convince them to stop, and if you cannot, you will have to convince a court to make them stop. A patent lawsuit can take over two years just to bring to trial, and can cost several million dollars. There are lenders who might be willing to fund your lawsuit, and there are attorneys and law firms who might take the case on “contingency” (you pay them only if you win, but generally more than if you were paying them unconditionally), but it will likely also take a great deal of time and effort.