Benitez admitted his frustration at the lack of transfer activity so far this summer on Tyneside.
And Benitez is looking to sign more players before the transfer window closes later this month.
The Magpies had kept tabs former Manchester City goalkeeper Willy Caballero and Chelsea forward Tammy Abraham, but the former moved to Stamford Bridge as a free agent, while the latter opted to join Swansea City on loan.
"I can read our fans saying "listen, now will sign a £25million striker". But if they are available, the price isn't £20million, the wages are not the same, and the commission of the agents is huge.
"It is very hard because the prices are insane", he said.
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"We missed some targets at the beginning and I think we are paying for that ..."
Benitez, however, admits it would be "very difficult" for him to walk away from the job - and he reiterated his commitment to the promoted club.
"I made a decision to stay because of a lot of positive things I could see starting with the fans".
"I am not happy but at the same time for me it's a challenge". "We are getting to more difficult matches so I am working hard to be a better player for this team. I'm very realistic. Let's be sure we do very well every game and we're out of the bottom three positions and then see where we go".
But now he is ready to establish himself as a Premier League player - and he is relishing the challenge of facing Tottenham Hotspur in United's curtain raiser on Sunday. I knew there would be a lot of competition, I just need to do my best.