The main difference lays in the DAC/ACD chips. The better resolution, dynamic range and precision, or short: quality, they have the more expensive they tend to be.
Two 24-bit ADC/DACs do not necessarily produce the same results. Their dynamic range could be a mile apart.
In addition (or only if the chain is 100% digital) comes how the signal is processed and at what bit precision. The higher bit-resolution (ie. 24 and 32 bits or if it uses floating points) will require a better fit processor with higher capability (or an additional floating-point math co-processor, FPU) as well as more memory.
Then there is a third factor, the quality of the electronic components themselves such as tolerance values and so forth. The better quality these components has the higher the price will be.
As an alternative to physical compressor you could also consider a software compressor for a DAW. There are many good ones which do the same fine job as a physical digital compressor.
However, some argue that analogue compressors combined with analogue tape/over-saturation gives the best result.