Totaljobs said on average people spent almost £150 on every interview, including a new outfit, make-up, transport and help with writing a CV.
The jobs website said its survey of over 4,000 jobseekers also found that three out of five did not manage a decent night's sleep before an interview and most found the process difficult.
Many of those polled said they feared never being able to find a job, or not having enough experience to secure work.
Two out of three older jobseekers said they felt discriminated against by a prospective employer because of their age.
John Salt, group sales director of totaljobs, said: "It's concerning to see the amount of money that jobseekers are spending to get themselves noticed, when they are not preparing in some of the most basic, low cost, but effective ways they could be.
"Simple things like researching the role and the industry will obviously count for a lot at interview stage, as will the way jobseekers come across generally to their prospective employer in interview."
The report said jobseekers would be spending hundreds of pounds if they attended several interviews a year.