A full evaluation for Dyslexia can be expensive, but it's worth the investment for two reasons. One is, you need a comprehensive assessment in order to guide intervention.
The assessment should really examine three different areas. One area is the component reading skills that make up reading. Not everyone with Dyslexia has difficulty in all the different areas of reading component skills and you want to know which ones are the most troublesome so you can target your intervention there. The second area of assessment is other academic areas, such as math or writing. Very often, children with Dyslexia have difficulties in these other academic areas as well. You need to have an assessment of all the areas that are of difficulty, so that interventions can be put in place for those as well.
A third area of assessment are the mental health problems that often go along with Dyslexia. ADHD co-occurs with Dyslexia about 25 percent of the time. If those aren't properly assessed, the attention problems can actually get in the way or interfere with a reading intervention. So you want to identify and treat those reading problems. The same way with anxiety or depression, those are mental health issues that can interfere with the implementation of a successful reading intervention. You want to identify those and treat those as soon as possible.
The second reason it's important to invest in an evaluation is for documentation. You want to be able to document the diagnosis and the types of interventions and accommodations that a child needs as early as possible, that's for both schools, and later for standardized testing situations. There are online evaluations that are available very often at a lower cost.
However, those evaluations are often not as comprehensive. They may not assess all of the reading skills that you need to assess. They most likely do not assess other areas of academic achievement, as well as the mental health issues. For documentation, you need to have a report signed by a licensed Clinical Psychologist to get those accommodations. Not all online evaluations provide those types of documents.