Page 199 of the DMG explains how new characters can start with magic items, even those they choose to make themselves (with the assumption these items were created some time before the character became a PC)
But let’s say a non-spellcaster wants to give themselves an extra point to an ability score. If they have 27,500 gp, they could start with a Manual of Quickness of Action and either use it immediately or possibly say they’ve used it already and begin with an inherent +1 bonus to Dexterity.
That book costs 27,500 gp. Using the formula on page 129 of the PHB, however, it would only cost 26,530 gp to have a 17th-level wizard cast wish on them as a service. For an 8th-level character (or a higher-level character with lots of other gear), that’s the difference between getting the bonus, and not.
Can the character simply decide to begin with this effect present (or a different spell with a permanent duration, like enlarge person+permanency, since 9th-level wizards are easy to find in large cities) and 26,530 gp less, or since the DMG only describes starting with magic items, must they spend 27,500 gp for the magic item?