I agree with Shannon, I think the first thing you need to understand if what is the goal for the SWAG you are selling. If the goal is a secondary revenue stream, and you believe it can be consequential towards your bottom line, then you can do the following:

1. Do some quick research on other startups/tech companies that sell SWAG to see where the market is priced at.

2. Do some quick market research with a survey to existing customers or even a random set of potential customers using something like Google Surveys (https://marketingplatform.google.com/about/surveys/) to get a quick barometer of the price point that feels plausible

3. A/B test different price points to understand your demand curve and maximize profit

4. Likely a good idea to price slightly higher than your ideal price and then offer a discount

If instead you want to maximize the volume of SWAG sold to help evangelize your customer base and drive awareness/growth, then you don't want to maximize profit you want to maximize sales while staying within your acceptable cost scenario (may be comfortable losing a little money on each sale as a marketing cost, may want to be break even, etc.). You may even want to charge a premium for purchased SWAG to subsidize additional SWAG that you give to free to your highest profile customers or best evangelists. At my previous startup we used SWAG as a "user love" tool, giving out SWAG to our biggest fans or as a parting gift when customers visited our office.

If you'd like to chat more let me know!