Cost of flights depends on where you are flying out from, but depending on the airline and if you book a few months in advance, tickets should range from about 250-600$.
Guatemala immigration requires you to fill out information when entering and exiting the country. Below is the link to access the form to fill out. Take a screenshot of the QR code to present to immigration control. https://farm2.sat.gob.gt/declaDelViajeroGt-web/pages/public/declaracionJuradaDelViajero.html
Antigua Guatemala is about 1 hour to an hour 30 minutes from the airport and can be reached via a private taxi from the airport or via Uber. An Uber would be about 40-50$. Price can change depending on type of vehicle/number of travelers/suitcase size.
The US Dollar equals about 7.5-8$ in Guatemala so 100USD would be about 800Qs. A majority of Guatemala accepts card payments and accept tap via options like Apple pay and Google pay. Check with your bank to see if there are foreign transaction fees. Most credit cards also do not charge foreign transaction fees and is the better form of payment. If you do not have a credit card then cash is recommended. If you prefer to bring some cash for cash only purchases such as tuktuks then carrying around 50-100$ worth of the currency should suffice.
If you are in the city center, everything is within walking distance. Shoe choice should be highly considered as the streets are cobblestone. Transportation in Guatemala is also very economical. There are TukTuks that are highly available that charge the equivalent of 3$ for a ride that is 5 minutes away. Uber prices are also not going to cost more than 10$ within the city of Antigua. Once you venture outside of the city, the prices will increase accordingly. Uber rides add more comfort compared to a TukTuk as the roads are made of cobblestone.
It is rainy season during the September time in Guatemala but the weather is still pretty warm. Being fairly close to the equator, the weather is frequently at 70s-80s Fahrenheit. Due to climate change it may or may not rain. Last year it did not rain at all during September and they experienced a bit of a drought(hopefully it is the same this year).
Antigua is a small historical town in Guatemala. It is also a very popular tourist destination so the costs of food and other items ends up costing as much as it does in the US depending on where you decide to eat/shop. For example a Coffee in Antigua costs about 4$ but Starbucks in the US would be about 6$. Also tipping in Antigua is accepted but most restaurants and bars will already include a gratuity %, so tip as needed if you'd like.