Benitez has made a number of signings, headlined by the £12million capture of Jacob Murphy from Norwich and Florian Lejeune from Eibar for £8.7m, but the Spaniard admits he is frustrated that more has not been done to improve last season's Championship winners ahead of Sunday's opener against Tottenham.
"I am not happy but at the same time for me it's a challenge," he said on Sky Sports. "I try to do my best."
Benitez said inflated prices in the transfer market were a big part of the problem, but also said the Magpies were paying the price for making a slow start to the summer.
"It is very hard, the prices are crazy," he added. "We missed some targets at the beginning and we're paying for that. Hopefully we can find solutions and bring what we need. It's not what you want, it's what you need."
When Benitez committed to stay at Newcastle at the end of last season he was promised "every last penny" by club owner Mike Ashley to invest in the squad, but the reality has not been so simple.
"When I decided to stay for me it was a challenge," he continued. "I'm not totally happy with what we did, but I will try to do my best. I'm working very hard with the people in the club to try to improve what we have at the moment."