From a person living in San Antonio and who uses Amtrak frequently.
First, regarding Amtrak, it is about 80% likely you will not arrive in San Antonio at 10:00 PM. Think more 11 or even midnight. Amtrak is notoriously late due to ownership of rails being in someone else’s hands other than Amtrak. You learn to just go with the flow and not get bent up. So go ahead and plan on being late. Perhaps very late.
Second, cheaper places to stay in downtown close to the station. Yes, there are hotels within biking distance from the station. But many of them, being downtown, won’t be inexpensive. There is a La Quinta just to the north of the station on Blum (which is just off Commerce St.). They publish room rates at $149, but many times you can ask/sweet talk/beg you way to a lower rate. This might be the cheapest you can find.
You might also consider calling the San Antonio Visitors Bureau at (800) 447–3372 and ask one of them about less pricey hotels around downtown.