The prosecution claims Mr Cas used the high-profile signings to suck in up-and-coming athletes, charging up to £1,000 a time to match them with non-existent sponsors through his Croydon-based Global Sponsorship Group.
None of the athletes received any money, and the company was shut down when staff contacted police over their own unpaid wages, the court heard.
Miss Boggis told the court she was lured into signing the contract – after lawyers gave it the all clear – because of the unprecedented amounts being offered.
The cash-strapped 21-year-old, who has been living in a caravan next to her training centre to save money, said she could not pass up a contract even thought it seemed “too good to be true”.
She said: “As long as the contract was fine and legal and everything was good, I honestly thought I can’t miss an opportunity like this.”
Miss Boggis told the court she was initially offered £15,000 a year up to the 2012 Olympics for taking part in photo shoots with the company, but the deal eventually rocketed to almost £85,000 a year by the 2016 competition.
She told the court she was also offered a £15,000 Audi, but never received payment or the car.
Mr Turner, the current European and Commonwealth Champion at 110m hurdles, told the jury Mr Cas offered him a £15,000 Audi car as part of his three-year contract.
He said: “Mark kind of swept the athletics world for a while, everyone was talking about it.
“We were sent a group email (by UK Athletics) to be wary of Mark because what he was offering was unheard of.”
Mr Cas denies five charges of making false statements to gain sponsorship contracts.
The trial continues.