Get the last 409A valuation. That's the price the shares should trade at and in fact will create problems for the company if they do not.

If the CEO offered the last 409A valuation as a price, then there's little you can do if you're inclined to take an offer because the 409A is the "fair market value" of the common stock. Usually the 409A is a huge discount, like 60-80% less at the stage you've implied the company is at, under the preferred price.

Consult a lawyer who specialises in startup financing.

Most people here have no idea what they're talking about.

If you are skeptical of the company's future, and the CEO is not to be trusted (and it is clear he is trying to take advantage of you), then ask yourself if you had 100k, would you invest it in this company? If the answer is no, then maybe you should take the buyout. On the other hand, how well do you live with regrets - if for example you were wrong and the CEO does not run the company into the round? If you really think the company will fail, then 100k is better than nothing.

How will the investors feel if they know that you would sell 5% of the company for $100k? Maybe this is your leverage: the investors will know the price of the buyout and since the investors know that you are very familiar with the company, then they will invest less. Therefore, the CEO should offer you a fair price for your shares.

Ask him how he came up with that price, see if he is able to make sense of it mathematically somehow, intuition is often wrong on pricing. Explain that a company with less owners (and thus less conflict of interest) is more valuable and less risky. Also a peaceful board is more valuable than a board backstabbing each other. If the CEO doesn't value these things then I am sure other investors will value it. You should be clear that he is stealing your cut of the value from risk reduction. Also, think if the CEO somehow has more information than you do. If he has more information and is buying, you should consider keeping the stock until relevant valuation info is shared.

Of course this could be wrong too, depends on the non-money terms the other investor got.

Who started the conversation? If the CEO did, you can sit back and tell him, "it's not enough." Rinse and repeat, and don't respond to "well how much should I be offering?" entreaties from their side. "It's not enough." Practice it in a mirror.

What leverage do you have?

Maybe by suggesting that you would contact the investors directly to purchase your shares at a slightly discounted rate?

If you know who the investors are you can contact them directly.

How old is your company?

You could say that if the CEO doesn't improve the offer, you're going to write directly to the board / new investors.

I'm not saying you should do this. Nor am I suggesting it's a good tactic. But it may provide leverage if you need some.