Equity in a business driven by software?

I am completely new to negotiating equity, and I don't know where I stand with regards to pushing back on potential offers of equity based on a new project.

If anybody could give me their view on my 'entitlement' based on the following context I'd really appreciate it:

I recently planned on launching my own software development start up and had some conversations with current business owners which, subsequently, led to conversations with somebody about to launch their own business based on an idea they've had.

This idea is predominantly driven by a software solution but I (my company) has only been offered 15% equity in the business (as well as payment for all of the time I will spend now 'for free' developing the product).

In the first instance, I was shocked at how low 15% was but, naturally, the other party proceeded to justify this equity with the fact that financial investors will want to seek shares in the company and that mine should never be affected, as well as the fact that there will be other directors involved in other key departments such as marketing.

Should a partner whose service is so integral to the success of a project be entitled to more equity or should I be grateful that I will be entitled to this amount without having to deal with marketing campaigns, politics, copyrighting and the fact that I'm not even established yet but have landed work?

I appreciate this is a potentially open-ended question not suitable for this site but what I'm basically asking is how can I value my worth in this company given this scenario?

The offer you have been given is confusing: "15% ... and your share will never be affected". Typically when people take investors in, they issue new shares, diluting all investors. (You start with 15 shares, they start with 85 shares, you take in new investors who buy 100 shares ... and the company issues 100 new shares, leaving your 15 shares out of 200 shares now being equal to 7.5%.)

The fact that they haven't explained this well leads me to say 'don't take the deal' because 'you probably don't want to be doing business with people who start a relationship based on deception'.

The offer itself of 15% of the shares plus a salary could be a very fair one, depending how big the salary is, what the company is being valued at, what the other people are doing, how big the team is, etc ... and more importantly when the 15% is measured. Do you have startup financing, so it is 15 shares to you, 35 shares to two other people doing good work and not drawing salaries, and 50 shares to cash investors? Then it's fair. Is it 15 shares to you and 85 shares to one other person who does nothing other than raise money? It's not fair.

15% equity and receiving $0 payment "I" would typically look at as a bad deal. But it always depends on the exact situation. If there is a new business being formed and the other partners are bringing in substantial cash to the business for sales and marketing then it may be worth it.

Example. Let's say after looking at how much development work needs to be done you value it at $50,000. If you were charging someone to build it then you would charge them $50,000. If the other partners bring $50,000 cash investment into the business instead and allow you to bring $0 because you are offering your time for free to build it then it may be worth it.

Typically software based startups are 90% software laboring and 10% sales / marketing for a long time. Once the product is mature then it can potentially flip flop where majority of the effort is sales and marketing.

But as a software startup, 99.9% of the work is making great software.