Many of the paid VST FX (Waves and UAD etc) are endorsed emulations of real analog Gear, Waves and UAD, for example, both have SSL Channel strip, EQ and compressor, or an 1176 or LA2A compressor, Fairchild and so on. These attempt to not just mathematically represent what a compressor, EQ etc does but also emulate the Euphinic distortion that these particular pieces of gear introduce to the signal that many find pleasing.
Given that the UAD and Waves emulations of the same SSL, 1176, LA2A etc models do not sound the same as each other (or exactly like the gear they are emulating either) it's unlikely they sound the same as other VSTs paid or free.
Since most plugins offer free trials (UAD needs a UAD DSP card, and waves and several others require an iLok, however) you can try them out and see which ones you like and if you like the difference between the free and paid VSTs. If you like what the paid ones offer then it's worth it if you don't then it's not. There are great paid and free VSTs and there are Lousy paaid and free VSTs too.