As thousands of fast-food workers prepare to strike over wages on Wednesday, much of their ire will be directed at the franchise owners who control the vast majority of the restaurants where they work. But franchisees have their problems, too.

A McDonald's milkshake machine costs $20,000. Its grills are $15,000. Making ends meet isn't so easy for either side, says Kathryn Slater-Carter. Until last month, Slater-Carter had been McDonald's franchisee for close to 35 years. She and her husband, Ed, got into the McDonald's business in the early 1980s in Daly City, California. Leaving behind jobs in education and banking, respectively, to join her father in the family business. Franchisees account for about 90% of McDonald's stores in America.

"Initially, we had three stores. My dad retired and sold one, so Ed and I ended up with two," she said. Then, a few years ago, McDonald's refused to renew a franchisee license for one of their stores. The store, in the local mall, brought about $2m a year, and provided jobs for about 30 people. Yet, just like that, it was shut down, Slater-Carter told the Guardian.

McDonald's, which collected rent worth of about 12.75% of each month's sales, did not renew the lease with the mall. The space, which Slater-Carter and her husband had spent $500,000 renovating a few years back after asbestos removal, had to be restored to its original state, which cost them another $10,000 in contractor fees.

Shocked by the way they were treated, they decided to sell the second location, where they employed about 55 workers and brought in about $3.6m in sales each year. The license on that store was valid until June 2016, but they weren't going to take any chances.

"McDonald's decisions about whether to enter into new franchise agreements with existing owner/operators are determined on a case-by-case basis. We discuss those decisions with our owner/operators, but do not discuss them publicly," Lisa McComb, a spokeswoman for McDonald's told the Guardian.

As a recent franchise owner, Slater-Carter is well positioned to talk about the challenges and the costs that franchisees often face.

Wages: corporate v franchisee

"My rough calculations – we don't have precise average wage information from McDonald's – are that the impact of a $1 increase to the US average wage is approximately $50,000 to $54,000 per company-owned restaurant," John Gordon of Pacific Management Consulting Group told the Guardian.

If franchisees were to implement similar wage hike, their cost per restaurant is likely to be higher.

"Franchisees traditionally pay a little bit less. If they were trying to make it so that they are a dollar over the minimum wage – state, local or federal – then the impact might be a little bit more," said Gordon. Average franchisee wages are not reported, so any estimates are purely a guess, he added.

As it stands, McDonald's franchisees are dealing with additional costs:

franchisees pay a royalty fee to McDonald's of about 5%, according to Gordon. Slater-Carter said their royalty fee was 3%;

they also pay an roughly 5% for advertising to McDonald's;

many of the franchisees pay rent to McDonald's. For Slater-Cater, that was about 12.5%. On average, Gordon said that number is about 15%;

remodelling costs can be huge and are required by the company for renewals.

With all these extra costs, franchisees are more likely to have a smaller profit margin than the corporate store and, thus, less funds to use for a pay hike.

"McDonald's does not set wages for the independent owner/operators," McComb told the Guardian when asked if the company would offer any assistance to franchisees looking to raise wages for their employees. "Independent owner/operators make decisions that they feel are right for their employees and their businesses."

Slater-Cater claims she has received advice on wages before. Last year she told the Washington Post that McDonald's representatives had told her to "pay your employees less" when she complained about low prices hurting her business.