For opening night, you will not be able to get a reservation, those have been sold out long ago, and as you mentioned, you can only reserve, and pay for food and beverages ahead, for 10.
I am not sure that paying hundreds is worth it either, not being snarky, but if it were, then I would have planned it into my trip.
The fest itself costs nothing to go, getting in the tents is also free, you just cannot have a beer unless you are seated. Tents will close for a time if they are at capacity, but you can still walk in, see the tent, catch a bit of music.
If there are open tables, with no reservation for that time, you are free to sit; if you are a bit gregarious and see a table of 8 or less people you might ask if you might join them. In some of the smaller tents, you might find a place to sit, maybe especially late afternoon, before the evening reservations start. In any tent, look for the waitstaff responsible for that section, they know if there are free tables.
All that said, I would say "go", see the tents, wander the grounds, see the spectacle, try to get a beer in a tent. The beer gardens in town will be hopping, so a good time can be found. If you will be there earlier in the day, there is a nice parade as well.
Not sure what the rest of your trip involves, but the Monday after opening weekend (the 19th) is a good day to go without reservation (or most weekdays); and Stuttgart has the Cannstater Volksfest, which many claim to be better than Oktoberfest.