Cost effective mods for a Sony SCD-777ES

I wouldn't touch it. Rather, I'd buy a good used DAC and use the Sony as a transport. Why? Because IMO the DAC...say a Dodson or similar...will outperform the Sony even after mods. Also, the Sony will retain more of its residual value as a stock piece as the Sony players become more and more rare.

Ditto Czbbcl's recommendations for modders. If you can do the work yourself and want to work incrementally, start with the Superclock 4, replace the AD712 op amps in the output stage, replace select caps/resistors in the output stage signal path, remove the AC filters, and star-ground each board. There are posts on AA SACD Forum regarding these changes.

I agree with Tvad and Drubin. Use the SCD 777 as transport (excellent one).
I use mine for transport and use an out board DAC with excellent result. With the cost of mods, you can get a good DAC such as Musical Fidelity Trivista, or a tube DAC easily.
This way also give you the flexibility down the road if you like to change the dac for different sound.

The Sony DAC is not a gating factor. With clock replacement, power mods, & a swap of output stages to VSEI or buffered RAM coupling transformers, the 777ES will approach SOTA.

SOTA has many different flavors these days as a quick listen to the variety of SOTA digital players will reveal. Using an external DAC is the easiest way to experiment with different flavors.

However, replacing the clock and upgrading the power supply on the Sony would be beneficial to it's function as a transport.