LastMinute.com review: Plane ticket reservation

I tried to reschedule my ticket through the agency where I originally booked it, but the system says LATAM doesn't authorize changes because my flight has already departed. On LATAM's website, however, it insists that I must make the change through the agency. I've been trying to resolve this since April, but the agency did not provide me with any reasonable offer, and, until right before my flight was taking off, but I couldn't fly and ended up losing the ticket.
LATAM is telling me they can rebook it, but only if I pay Airline penalty: 174.00 EUR
- Price adjustment: 2019.00 EUR
- Handling fee: 35.00 EUR
- Total cost of change: 2228.00 EUR
- on top of what I already paid. What doesn't make sense is that their one-way more than double of the price of a round-trip tickets at the same date and airline. which makes the rebooking cost seem completely unreasonable.
Am I misunderstanding something here, or is this pricing structure simply illogical?