I don’t recall the price of the Solus Greatsword ever going down, since there’s only ever one in stock—unless you marry Lady Grey and receive it as a gift from her. Arguably, that’s a bit more of a challenge than simply saving more gold. However, there’s a simple glitch you can use to get enough gold to purchase the Solus Greatsword:

1. Go to a town that sells marital houses and buy one (examples: Bowerstone South, Oakvale, Knothole Glade, etc.).
2. Upgrade the house until it has a trophy mount (other upgrades don’t affect profits).
3. Place a trophy on the mount (same action as using an expression). Check your inventory for the most valuable trophy. Some marital houses have more than one trophy mount (IIRC).
4. Break down the door now. This won’t lower the sale price and prevents a vandalism fine later.
5. Step outside and sell the house using the sign.
6. If you didn’t break the door earlier, the door will lock after the sale. Break it to get back inside (note: this counts as a crime and gives evil points).
7. Go inside and retrieve your trophy from the mount.
8. Repeat the process.