Hello,
In October in Paris, take place many exhibitions, conferences and shows.
The website Viparis gives the calendar of all these events.
During this period, Paris may be overbooked and higher prices for hotels rooms are charged.
If you book in advance, you will benefit from low rates. Otherwise during exhibition time you cannot find last minute deals, on the contrary, the same room can be sold 3 or 4 more than the real price.
The hotel des Carmes is very well located, near Notre Dame and benefits from a perfect quotation, not expensive at all.
As far as hotels in Saint Germain are concerned, The Madison Hotel is well known, as well as the Hotel Sainte Beuve, the Hotel D'aubusson (4 star), the Hotel Millesime, the Hotel Louis II.
I should advise you to find a convenient hotel for you, and book it now, you will save money.