Most changes can be made with one flash, true. However it depends on what the problem is, if any, and how much rooting around is deemed appropriate to customize the car to suit you. Unless you are looking to maximize WOT performance at top revs, paying extra for dyno time is overkill. Having said that, safely measuring output and AFR during PE can only be done with a properly setup dyno, operator and tuner. That's at least two people, a shop, a PC driven dynomometer, and miscellaneous props.
But no matter who does the tune, most time is not actually on the dyno or driving but calibrating after reading, scanning, logging and analyzing.
More so than rollers a good street tune requires WideBand feedback. Without that it is fairly hit and miss, and I imagine gumtree and eBay dudes utilize vehicle sensors only, with a FlashScan or other handheld.
Consider the actual seat licence for your VIN is about $200, call out $200, $150/hr for time spent. That's about cheapest you can get from someone who speaks English and doesn't resent doing the job because you are being scabby and there's no money in it. Definitely will be better than stock, restricted on what can do without WB or rollers.
If you are getting CAI / exhaust you are way better off paying a couple of hundred more to get a full custom tune (that's if you want max WOT performance), book it in with a tuner who at least has access to the the gear above and do it once.