Rafael Benitez admitted he is "not happy" with Newcastle's transfer business so far as they prepare to make their Premier League return this weekend.Benitez has made a number of signings, headlined by the £12million capture of Jacob Murphy from Norwich and Florian Lejeune from Eibar for £8.7million, 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," Benitez 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 said. "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 said. "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."However, the 57-year-old deflected a question regarding his future with a joke."It's very difficult for me because my Chinese is not good enough," he said, referencing links with the Chinese Super League."I am trying to do my best right now. It is something you cannot control but at the moment I am fully committed to doing the best that I can."