I don't have much in the way of figures on hand for cost but I did do a write up on the cost of a Knight's equipment in the thirteenth and fourteenth centuries a little while ago. That write up draws exclusively from English evidence, though, as I'm vastly more familiar with that than I am with the French evidence.

One thing I think is particularly interesting with regard to this question is an entry for Edward II in 1312 where he bought two swords simultaneously. He bought them along with a haubergeon and some repairs to equipment he already owned. Now this is a king so he's a bit of an outlier but we also have the case of Fulk de Pembridge who died in 1326 and left a suit of armour to each of his three children so redundant equipment was certainly not the exclusive right of kings.

Slightly closer to the time and location you're asking about (But still not quite the same) is the inventory of Raoul de Nesle. He was a French knight of great importance, he was chamberlain of France and joined Louis IX on the 8th Crusade. He died in 1302 at Courtrai and left a great inventory upon his death which includes extensive references to weapons and armour. His inventory appears to include something like 35 swords which is rather a lot assuming I'm correctly interpreting all instances of '[Roman Numeral] swords.' Even if all those extra numbers are ignored there are 8 individual entries devoted to swords. He had a lot of swords is what I'm saying. Also, if you're interested in medieval arms and armour trivia this is probably the first reference we have to medieval plate armour since his inventory includes references to greaves.

As to the frequency with which knights would ride around with multiple swords...that's up to someone else. I honestly don't know. I'm also lacking in good early 13th century sources for swords right now. Sorry!

Prestwich, Michael, 'Miles in Armis Strenuus: The Knight at War', Transactions of the Royal Historical Society, Sixth Series, Vol. 5 (1995). 201-220

Kelly, Francis, "A Knight's Armour of the Early XIV Century Being the Inventory of Raoul de Nesle" The Burlington Magazine for Connoissuers Vol. 6 No. 24 (1905)

Although /u/Valkine might wish to add to it, this is an excellent breakdown of the cost of military arms and armour.

Jean de Joinville was a prestigious noble in thirteenth-century France, after he lost his war-chest for the campaign (after he was captured by the Saracens) he received a grant of 20,000 livre tournois which covered the replacement of his arms, armour, horses, and to resupply and pay his own men.

It wasn't uncommon for nobles and knights to have multiple horses, which could cost the income of a village (£40 p/a), to have a few swords wasn't anything particularly special.

Side question; does anyone know where I could purchase a physical copy of this biography? It seems fascinating.

I made a comment earlier, but I've just remembered a video which may be somewhat helpful to you in regards to the price of swords. Here it is.