Paying a stranger to read your SOP feels weird. It is your story. But here is the thing I keep coming back to: the admissions committee are strangers too, and they will spend about four minutes on your file. Four minutes. That is the whole audition.
A good SOP review service is not selling comma fixes. It is selling a stranger's eyes before the real strangers get you. The useful question is never "is my grammar fine." It is "does my first paragraph tell them why I am here" and "would a tired professor remember one line of this at 11pm." If the answer is no, you need structure work, not proofreading.
The options fall into rough buckets. Alumni reviewers who got into your target schools, and who know the weird unspoken preferences, like which departments secretly hate flowery openings. Professional editors with admissions experience, strong on structure, just check they know your field. Freelancers, which is a lottery. Some are excellent. Some run spellcheck and invoice you. Peer review communities, free and honest, but wildly inconsistent. And statement of purpose review app options, which are fast and cheap and genuinely decent for early drafts, when all you need is someone to point at the tangled sentences.
My take, after watching people do this every which way: use the free and cheap layer for drafts one through three. Then pay a human once for the final pass. One round of serious feedback beats five rounds of light edits. That single pass is the highest-leverage money in your whole application.
Some people prefer a dedicated SOP editing service for that final pass. Either way, make sure the last version is shaped around a specific university. The tailoring packs at sopexpert.fyi build the draft around your target school from the first sentence, with statement of purpose writing help folded in.