What is the big difference in Botox prices?

I have been looking for clinics lately as I consider getting Botox, but the prices vary so widely that I don't know what criteria to use when making a choice. Some places offer really cheap promotional prices, while others charge quite a bit for the same Botox. I'm wondering if this is due to the type of product or dosage, or if it's because the treatment method is different. Since this is my first time, I'm worried I might regret it if I decide based solely on price, and I'm also debating whether getting a consultation first is the right decision. I'd like to know if those who have received Botox were satisfied even at cheaper places, or if it was better to choose a clinic with experienced staff, even if it costs a bit more. I'd also like to know if the duration of the effects varies by clinic, and if there are any cases where additional costs are incurred. Please share your experiences. Since this is my first time getting Botox, I'm very concerned about even the smallest details.