Current proposed manned Mars mission plans are very diverse and which plan you chose would affect whether to include a return trip. Let's weigh some pros and cons:
Pros (no return trip)
Save on fuel transport costs OR
Don't require ISRU fuel factory
Spaceships used once and can be constructed cheaper
Cons (no return trip)
No sample return means heavy lab equipment needs to be sent (Earth labs are very advanced)
Habitats need to last forever or for a much longer time
Moral/ethical/PR/medical implications of stranding people on Mars forever
Taking these factors into consideration, a short duration mission as frequently proposed by NASA would probably be cheaper with a return trip as designing habitats and equipment to last for a couple months is far easier than designing a habitat to last forever. You could also save tons of mass by bringing little scientific equipment and instead just bringing sample collection equipment.
If you're looking to establish a permanent colony, and have the money to do so, I still think that return trips are worth it. People being away from Earth might be very damaging to the psyche in the long term + the effects of low gravity on the human body are still unknown. Additionally, the costs you save by not including a return trip don't scale that well as the major difference is that you need some sort of ISRU for fuel production on Mars. Throwing away the transport spaceship every trip can't be cheap. Launching from Mars is also easier due to low atmospheric pressure and lower gravity. A Mars economy also needs to make money if you're planning to make it permanent and no return trips removes lots of people from the willing astronaut pool of people and basically kills any Mars tourism.