Is this cheaper than buying a SaaS like Inventory Planner or Lokad?

Long-term, yes. Short-term, it depends. SaaS economics: Inventory Planner is $150-500/mo; Lokad is $1.5k, 5k/mo; RELEX / o9 are enterprise-only with 6-figure annual contracts. Build economics: 2 weeks of senior Python work (~$5k) + a few hours/mo of maintenance (~$200/mo). The custom build pays back the SaaS license within 6-18 months at Inventory Planner pricing, and within 1-3 months at Lokad pricing. The non-financial trade-off: SaaS tools come with support, training, and a working product on day one. The custom build needs in-house Python ownership forever. If you have a data team or a strong Magento dev, build. If not, buy.