Quick Answer: Buy Used First, Not the Cheapest New Robot
If you need a robot vacuum under $100 for carpet, the most realistic answer is a used or refurbished Roomba 600-series, especially a Roomba 675, 690, or 694. These older iRobot models are not fancy, but they are simple, durable, easy to find parts for, and better suited to carpet than many no-name budget robots that look tempting on Amazon.
For a tiny carpeted office around 120 to 150 square feet, you do not need mapping, mopping, self-emptying, or app-heavy navigation. You need a robot that can move reliably, agitate carpet fibers, return to its dock, and accept cheap replacement brushes, filters, and batteries. That is where a clean used Roomba often makes more sense than a brand-new bargain model.
Best Pick Under $100: Used Roomba 600-Series
The Roomba 600-series is old-school: bump navigation, a side brush, a 3-stage cleaning system, and Dirt Detect on many versions. It is not graceful, but it is proven. On Facebook Marketplace, eBay, or local refurb sellers, a working Roomba 675, 690, or 694 often lands around $60 to $120 depending on battery condition and included accessories.
The biggest advantage is repairability. Replacement rollers, side brushes, filters, wheels, batteries, and bins are widely available. For a small carpeted office, that matters more than a slick app. If the robot runs daily or every other day, it can keep visible debris under control without needing high-end mapping.
Before buying used, ask for a video of the robot leaving the dock, vacuuming for five minutes, and returning to charge. Check that both main brushes spin, the side brush is not stripped, and the battery lasts at least 45 minutes. Budget another $15 to $30 for a fresh brush and filter kit.
Best New-ish Alternative: Eufy RoboVac 11S Max
If you prefer buying new, the Eufy RoboVac 11S Max is the first model worth watching for discounts. It is usually above the strict $100 line, commonly closer to $130 to $180 new, but sales and open-box listings can bring it nearer to the budget. Eufy lists the 11S Max with 2000Pa suction, a slim 2.85-inch body, BoostIQ suction adjustment, self-charging, and support for hard floors to medium-pile carpet.
For carpet, the Eufy is best on low to medium pile and smaller rooms. It is quieter and slimmer than a Roomba 600, which helps if it needs to clean under a desk or low shelves. The tradeoff is that it uses random navigation and does not have Roomba's long repair ecosystem. It is a pleasant simple robot, but at the same price as a healthy used Roomba, the Roomba is usually the safer carpet buy.
What About ILIFE V3s Pro and Other Cheap Robots?
The ILIFE V3s Pro and similar budget robots can be decent for hard floors, pet hair, and very low-pile rugs. ILIFE markets the V3s Pro for hard floors and low-pile carpet, with simple scheduling and self-charging. The problem is that these machines usually rely more on suction than brush agitation, so they are less convincing on a fully carpeted room.
That does not mean they are useless. If the carpet is thin commercial office carpet and the main mess is dust, crumbs, and hair, a simple ILIFE-style robot can help. If the carpet is plush, dark, thick, or full of tracked-in grit, it will probably disappoint. At that point, even a cheap upright or stick vacuum will clean deeper.
What to Avoid Under $100
Avoid unknown brands that advertise huge suction numbers but have few replacement parts. Also avoid mop-combo robots in this price range; the mop feature adds complexity and usually does nothing useful for a carpeted office. If the listing does not clearly show replacement filters, brushes, batteries, and wheels, skip it.
Be careful with robots that have no physical remote or basic scheduling unless you are comfortable relying on an app. Cheap app-connected vacuums can become annoying if Wi-Fi setup is flaky or the manufacturer stops supporting the app.
Simple Buying Checklist
For a carpeted office under 150 square feet, prioritize these basics: replaceable brush rollers, available batteries, a dock that works, enough clearance under furniture, and a dustbin that is easy to empty. Mapping is optional. Mopping is irrelevant. Self-emptying is outside the budget.
The practical recommendation is straightforward: buy a used Roomba 600-series if you can inspect it or get a clear test video. Buy the Eufy RoboVac 11S Max if you find a real sale and want something quieter and slimmer. Only choose ILIFE V3s Pro-style models for very low-pile carpet or mostly hard-floor spaces.
Bottom Line
The best robot vacuum under $100 for carpet is usually not a new sub-$100 robot. It is a clean used Roomba 675, 690, or 694 with healthy brushes and a decent battery. For a tiny office, that gives you the right mix of carpet pickup, parts availability, and low-risk ownership. If you cannot find one locally, wait for an Eufy 11S Max sale instead of gambling on a no-name robot with poor parts support.