If you don't have the Enchanting skill yourself I'd go with the cost of a Power Focus (MitS 169). There are advantages and disadvantages to a metal caricature power focus that is automotive but at the same time permanently active and vulnerable to astral attack, Lone Star patrol, etc. Bonding it will eat your starting spell points at 5kp/Force.
Otherwise get Enchanting 6 (and might as well get Talismongery 6 (especially if using the optional rule where you get a number of levels in BackgroundSkill equal to ActiveSkill-3 for free)). With your starting funds buy an Enchanting Shop (Alchemy), Kit (Talismongery) (MitS 169) and radicals (MitS 170). This will provide a nuyen cost for your homunculus which is what you were looking for. You could buy Ally conjuring materials with your starting funds too if you want to be economic. Read MitS 40+ about creating your own foci to decide what radicals and how many to buy. You'll then either need to buy Lodge Materials / Hermetic Library or pay 3kp + (resources/#members)
to join a Magic Group that has a Lodge or Library you can use. Then you generate your Ally Formula (MitS 111). Then you create the Focus Formula (MitS 42).
Inhabiting on p108 has more detail. Then you artifice your focus and bond it using the reduced first bonding costs, saving the remainder for buying spells. You now have an empty homunculus at chargen.
Designing each formula and artificing the focus all require time. Depending on the gm, this time may cost nuyen since lifestyle, contact upkeep, vehicle maintenance and magic group membership are all ongoing costs.