A full professional edit for a novel can run anywhere from a few hundred pounds to several thousand, depending on length and which service you use. For a first-time self-publisher, that's often simply not available, and the advice to "just hire a professional editor" isn't wrong so much as unhelpful — it doesn't tell you what to do with the manuscript sitting in front of you right now.
The useful thing I've learned across eighteen books is that "editing" isn't one job. It's four distinct passes, done in a specific order, and each one requires a genuinely different skill. Once you separate them, it becomes obvious which ones you can do credibly yourself and which are worth saving for.
1. The developmental edit — big picture. Plot, pacing, character arcs, whether the middle sags, whether the ending earns what came before it. This is the hardest to do on your own manuscript, because you already know how the story is supposed to work, which makes it nearly impossible to see where a reader would actually get lost. The best free substitute is time and distance: finish the draft, put it in a drawer for four to six weeks minimum, and read it again as close to cold as you can manage. A small panel of beta readers in your target genre — three to five people, given specific questions rather than "what did you think" — will catch most of what distance alone won't.
2. The line edit — sentence level. Rhythm, word choice, showing versus telling, cutting the sentences that are doing nothing. The single most effective free technique here is reading the whole manuscript aloud, or running it through text-to-speech and just listening. Clunky construction that your eye slides straight past on the page becomes obvious the moment you have to say it, or hear it said.
3. The copyedit — consistency and correctness. Grammar, punctuation, and continuity: did a character's eye colour change halfway through, did two days pass when the text implies one, is the timeline internally consistent across four hundred pages written over eighteen months. This is genuinely difficult to self-catch, because your memory of what you intended to write quietly overrides what's actually on the page. A running style sheet — character details, timeline, world facts, kept separate from the manuscript itself — prevents most of these errors from ever being written in the first place, which is a large part of why I built that kind of tracking directly into The Bindery's Cast and World bibles rather than leaving it to a spreadsheet nobody remembers to update.
4. The proofread — last pass, fresh eyes. Typos, formatting slips, the odd doubled word. This should be the very last thing that happens, ideally by someone who hasn't read the manuscript before and isn't reading for story at all, just for surface errors.
If I had to spend money on exactly one of these four, it's the copyedit. Structural problems and clunky prose cost you engaged readers gradually; visible typos and continuity errors cost you trust and reviews immediately, on the first page a reader sees. Do the developmental and line passes yourself, with distance and beta readers doing the heavy lifting — then put whatever budget you have toward a professional set of eyes on consistency and correctness before the book goes anywhere near a retailer.