The war monument ends up earning you almost as much pron as the mage guild would if you don't buy the givini tunnels before gawnfall from the massive boost to new givini country score it gives pushing the orc merchant returns up a tier, it also gives a huge boost to your social score right at the time you need to aim for 40 social, and gives a notable boost to takkan score too which helps with the takkan vote.
That said, if you're not planning to open the givini ruins before the war, you can afford a lot more investments, none of the boosts the war monument gives can't be gotten elsewhere, and you can always get all the hidden scores from the war monuments buying them later, even in chapter 5, so I don't buy the war monument before chapter 5 in an extremely optimized route where I don't open the ruins before the war since I can hit the orc merchant tiers without it because of all the extra ProN from not opening the ruins early and the mage guild does bring in a not completely insignificant ProN income. Usually nets an extra 100k ProN at least by chapter 5 compared to the war monument if you have enough ProN you don't sacrifice anything else making the trade. It'd earn more but you lose a bit of income from having to spend more on social investments instead of profitable investments. round 1.
The war monument is just really, really cost effective if you don't have that massive amount of Pron to play with and need to be more economical to have enough to open the ruins before the war.
Honestly I suspect I'll switch to buying neither round 1 in a more long term optimized route in the future, its impact on Donovan's suitor score likely isn't going to matter in the end and you can earn a lot of ProN by putting them off until later.