The Loudbox costs $6000 and contains 4x n300 cards at $1400 each so on paper you only pay $400 for the box with 2 Xeons, 3.8TB NVMe SSD and 512GB DDR4-3200 ECC RAM.
Some buyers of the loudbox might be tempted to sell the four n300 cards separately and keep the box, i'd say it's worth at least $2500 or so.
How fast are the n300s compared to RTX 4090 or RTX 3090 cards?
At these prices it doesn't seem worthwhile to stick them into your own workstation instead of going for theirs.
Don't beat yourself up about it. George Hotz thought NVIDIA Inception pricing for Ada RTX 6000s was $1,500, instead of $1,500 discounts, and launched a whole company based on the misunderstanding.
How fast are the n300s compared to RTX 4090 or RTX 3090 cards?
At these prices it doesn't seem worthwhile to stick them into your own workstation instead of going for theirs.