SLIM is a $100m project, including half of the launch(rideshare with XRISM x-ray telescope on H-IIA), not a billion dollar JPL project on a dedicated Atlas V, so budget likely is a factor.
Any source on the mission cost? I'm quite curious. The best Wiki has is a page from 8 years ago that gave an estimated 'cost of development' (which is unclear if it includes e.g. launch costs) of $121.5 million. [1]
The "Budgets" page in JAXA website([1], will quote below) indeed don't have a detailed breakdown:
| Project name | Total development cost(projected) | Total development cost(as of Jan, Reiwa 5) | Planned launch fiscal year | Project phase | State of Project (total dev. cost, launch FY changes, etc.) | References |
| Small Lunar Lander Proving Craft(SLIM) | 180 oku-yen(~$121m) | 149 oku-yen(~$100.7m) | FY Reiwa 5 (FY2023) | Phase D (Production/testing phase) (snip) | March, Heisei 28: Project migration review at JAXA (...snip...) March, Heisei 30: At JAXA, changes of plan due to change in launch vehicle(Epsilon -> H-IIA rideshare), as well as change in launch dates. Incorporating the results, adjusted total development cost. (180 oku-yen -> 149 oku-yen) (...snip...) | (snip) |
There are few more media sources[2][3] that state 149 oku-yen[4] figure covers "part of the launch and initial operation". One of such articles[3] estimates Epsilon launch cost as 50 oku-yen or ~$35m, and theorizes change to H-IIA to be intending to save launch cost. Not sure if there are readily available English source, sorry for that - very few of us think in English and these deep topics rarely have English coverage.
On budget, but at quite a small one for what it is. Epsilon all-solid launcher had anomalies and changes in these years too, so that could also be a reason for LV change.
[1] - https://en.wikipedia.org/wiki/Smart_Lander_for_Investigating...