The main expense in procuring a patent is legal fee. Each professional has their own hourly billing rate, which is governed by the basic principles of economics — supply and demand. “Cheaper” professionals usually have a larger failure rate than other reputable professionals. If an application is not written correctly there is no way to fix it later.
Generally, it takes 2-4 years to procure a patent, and one should never select a professional based on their initial filing fee, but on their success rate. Always ask for issued patents as samples.