The cost of roadside charging is rather in a state of flux at the moment, so there is no simple answer to your question. If you had asked the same three years ago when most of us here bought our PHEVs, the majority of public access charging points were effectively free to use - typically you just paid a small annual subscription fee and the electricity was free. Under those circumstances, roadside charging was very cheap - cheaper even than charging at home. The situation is changing a lot now - most of the charging network operators are introducing per kWh pricing - and often at prices that make it more expensive than simply running on petrol.
It is debatable if charging via the engine ever makes economic sense - the generator and battery are good, but still a fair bit less than 100% efficient, so there will always be power losses and if you want to minimise running costs, you are probably better off simply letting the engine drive the wheels via the generator and electric motors. Under normal driving conditions, there is nothing to be gained from burning petrol to take a flat battery up towards full and then consuming the charge in EV mode.
Your best strategy is probably to charge the battery wherever you have access to electricity at domestic rates or better, then manage the use of that charge across your day. If your journey is within the EV range (which, realistically, is closer to 20 miles than 30). then just drive normally. If your journey is going to be significantly longer than the EV range, then use the Save button to reserve the charge for those parts of the journey where you will get the most bang for your buck - in other words, the slower, flatter parts of the journey. A fully charged battery will get you further at a steady 30mph on a flat, clear road than it will at 70mph on the motorway, or climbing a long, straight hill - use Save to ensure that you are burning petrol under those conditions.