OLX-Dedham + GLX-Needham were studied multiple times over., so there are previous cost metrics to lean back on there. It should be cheaper than GLX because of the outright mode conversion rather than augmentation + widening, and should not have any buried blowouts. The Green Line half in particular should come pretty cut-rate because as a D Line appendage it would have minimalist non-prepayment stations, leaving only the 4-5 OLX stations as any sort of substantial production. Those savings more than absorb the cost of the Newton-Needham ROW reactivation, new Charles + 128 bridges, and slightly under half-mile of rail-with-trail accommodation. Just don't try to force-fit anything past VFW Pkwy. on OLX on the core build, because the Needham Cutoff and 128 were never benchmarked previously.
For the OLX stations, Rozzie Sq. would be a big one needing substantial busway...but the land is already available if they trade the parking glut for facilities space. Bellevue and Highland are in-the-pit station analogues like the lesser GLX intermediates, and W. Rox is an East Somerville analogue for being perched on a side-street overpass. But with no side-by-side or path accommodations required at any of them. The probable VFW Pkwy. terminus (kind of a gimme add since it's en route to the storage yard) can be a little sprawlier with parking because Home Depot is the only land barter required for that add. The ROW, while very tight-looking in spots especially in the pit around Highland/Bellevue, was indeed double-track into the 1950's and tri-track between Forest Hills and Rozzie Depot. It only looks narrower because of the re-centered track and an additional top coat of ballast dumped on during the 1986 line rebuild to create better side drainage. No structural touches required...probably just some catchbasin installs on the sides for equal-or-better drainage @ DT width.
I mean...if this Admin. ever took up a re-study I *fully* expect it to be a tankapalooza exercise in futility booby-trapped with all sorts of "WTF?"-level blowouts. Because that's just how they roll, and it mutually serves to throw a little more shade at the Purple Line's Rail Vision while they're at it. But much like the last Red-Blue study it won't pass the laugh test, because the cooked books won't track the cost inflation with well-sourced data from previous studies in any rational way. Whatever is variable with Needham conversion costs, it's a substantial cut below GLX's level of complexity. It'd be the sort of thing where if they try to bury a half-billion in "contingency" estimates it gets thrown right back in their face because not even worst-case inflation traces a blowout that large from previous studies...exactly how they ended up backpedaling so quickly from the last Red-Blue tank attempt. So long as they're hellbent on trying to knife the Rail Vision you'll never hear one word uttered by a state-level flak about Needham rapid transit, but if they do indeed go big with the Purple Line service investment Needham Green/Orange transactionally isn't all that outsized if you view it as a de facto cog in that sprawling regional plan.