You're correct. The vessel & primary heat exchanger of a MSR which are in contact with liquid-fuel is equivalent to fuel assemblies of many solid-fuel reactors. In solid-fuel reactors the clad which is designed to contain fuel doesn't last long. Similarly, in MSRs the materials in contact with fuel doesn't last long.

So, we need to dispose the vessel and primary heat exchangers similar to Zr-clad million dollar fuel assemblies.(PWR/BWR reload costs $60-100 million per 12-18 months.) Material wise Zr clad is way more expensive than stainless 3XX & graphite combined. Even if Ni-alloy is used in disposable MSRs Ni-alloy+graphite will be slightly cheaper. (Base metal costs: Zr metal: ~$22,700/ton Ni metal: ~$13,100/ton)