The resistance value when the photorelay is ON is defined in the technical documents as "ON-State Resistance RON". Toshiba has a lineup of products with resistance values in the range of tens of mOhms to tens of Ohms. Note that when it is OFF, nothing is specified as a resistance value. However, it can be found by converting a value of "OFF-State Current IOFF" defined as leakage current into a resistance value (calculated by (VOFF/IOFF). Although it is in a range of several tens of MOhms, you must be aware that as one of the characteristics of semiconductors, the OFF-state current IOFF largely depends on temperature.
Example: Case of photorelay TLP176AM