You dont need hydraulics for a grabbing claw. But thats the strongest thing you can have. But as Stu says, It is hidiously expensive, and you need lots of real custom metalwork to get it inside the weight.

A linak is the cheapest solution, but not the lightest nor powerfull.

Full Pressure pneumatics are an option, if you can afford it, and can prove youre capable to work with them. Its about the most dangerous option you have. But if executed well.. Look at Tough As Nails.

Otherwise, low pressure pneumatics are easier and cheaper to gather, but have less power to weight.