Climbing Cotopaxi with prices
First of all I must mention that it is very important to acclimatise before climbing Cotopaxi (5897m high), in order to increase your chances of summiting. I strongly recommend climbing one or two smaller mountains prior to Cotopaxi.
I''m trying to offer you the cheapest, safest way to climb Cotopaxi but by climbing on your own, the prices go up considerably. Cotopaxi
How is your level of spanish? As bilingual guides work out a bit more expensive. Below is an outline of prices for you to have a look at:

Spanish speaking guide:
1 person: $340
2 people: $250
Includes: Guide, transport & food up the mountain (dinner, breakfast & lunch).
Excludes: Refuge cost ($22), National park entry fee ($10 foreigners / $2 nationals), equipment hire ($15 rough estimate for two days as it seems you have a lot of the things you need already).
Bilingual Guide:
1 person: $395
2 people: $270
Includes: Guide, transport & food up the mountain (dinner, breakfast & lunch).
Excludes: Refuge cost ($22), National park entry fee ($10 foreigners / $2 nationals), equipment hire ($15-rough estimate).
As far as buying mountain gear in Cumbaya/ Tumbaco, there is a shop in Centro comercial esquina, Cumbaya right underneath Corfu (ice-cream parlour). Otherwise there are five good shops in the mariscal area (Quito) such as: Tattoo or Mono Dedo, you may find the prices a bit more competitive. In the mariscal area there are also various equipment rental shops, which is where we normally take our clients.
Cotopaxi is a mountain that can be climbed all year round so there should be no problem at the end of October. Having said this, the weather is very unpredictable in Ecuador, you can go in August when the weather should be clear and have terrible weather and likewise in the middle of winter have a strike of luck and have a clear day.