Like bogart said, the eight week scan is mostly to confirm the heartbeat (a $1 pregnancy test can't do that) and primarily to get the most accurate due date. Later in the pregnancy this gets much more difficult to do accurately.
You are one year away from AMA (advanced maternal age) status. Some of the more advanced genetic testing is only covered after 35, but it really depends on the plan. The early blood test at ca. 10-12 weeks is the newest thing. I think the rack rate for that was around $400. They claim that it is 98 percent effective. And while I do agree that people do with the information what they want, one of the primary reasons to take the earliest test is if you want to terminate. The 20 week ultrasound will likely flag all of the Trisomy disorders, as will later ultrasounds, but it's much more difficult and painful to terminate later. We knew we would carry whatever baby to term, so I figured 20 weeks would be enough time to mentally prepare. But this is a highly individual and emotional decision. Do whatever makes you most comfortable - early tests or no tests.
If your lab tests for STDs are going to be expensive on your plan (and they really can be, mine were over $500), you can also preemptively get your STD testing done at Planned Parenthood for much cheaper. I have three kids and therefore have been tested for HIV, herpes, etc. three times over the last six years. Overkill, IMO, but I understand why they have the rules. At least in my state, they have a lot of required STD testing, and you can perhaps decline, but the hospital will probably be ultra paranoid with you if they know you declined an HIV test and might even whisk your baby away post-birth for various testing reasons.
At some point with a baby, you have to just let the cost go. When I was 20 weeks last January, I was having trouble breathing. Because of this, my OB rightfully sent me upstairs to get some very expensive tests to be sure I didn't have a pulmonary embolism. We reached our high deductible (over $3,000) in less than two hours. Thankfully I was fine. It was frustrating to spend that money, but that just meant for the delivery we had already reached our deductible and had to pay just 20% of the bill. Don't hesitate to go to labor and delivery if you don't feel the baby move or have anxiety, just because of cost. It sucks to pay, but it would suck even more if cost held you back and you didn't get needed care quickly enough.