CSD Photo Booth — Cost per Poster and Event Budget
Every visitor poster is produced by two calls to Google’s Gemini image API. That API is effectively the whole cost of running the booth; everything else rounds to nothing.
Currency is marked on every figure: HK$ for Hong Kong dollars, US$ for United States dollars. Google bills in US dollars; HK$ figures are converted at 7.8.
The two models
| Model | API name | Role |
|---|---|---|
| Gemini 3.1 Flash Image | gemini-3.1-flash-image |
The default — faster and cheaper |
| Gemini 3 Pro Image | gemini-3-pro-image |
Higher quality, slower and dearer |
Output resolution is a separate choice, 1K or 2K. It changes the price on Flash but not on Pro, which Google charges the same for either.
Cost per poster
| Model and resolution | Time per poster | Cost (HK$) | Cost (US$) |
|---|---|---|---|
| Flash 3.1 · 1K — current default | ~22 sec | HK$0.54 | US$0.069 |
| Flash 3.1 · 2K | ~25 sec | HK$0.80 | US$0.103 |
| Pro 3 · 1K or 2K | ~43 sec | HK$1.10 | US$0.141 |
The setting is switchable at the booth, so it can be chosen on the day — cheaper and faster while a queue is forming, higher quality when it is quiet.
Event budget
| Posters | Flash 3.1 · 1K | Flash 3.1 · 2K | Pro 3 | Budget (Flash 1K + 30%) |
|---|---|---|---|---|
| 100 | HK$54 | HK$80 | HK$110 | HK$70 |
| 500 | HK$270 | HK$400 | HK$550 | HK$350 |
| 1,000 | HK$540 | HK$800 | HK$1,100 | HK$700 |
| 5,000 | HK$2,700 | HK$4,000 | HK$5,500 | HK$3,500 |
| 10,000 | HK$5,400 | HK$8,000 | HK$11,000 | HK$7,000 |
The last column is the one to budget against. A full recruitment day of roughly 500 posters should be budgeted at about HK$350.
Why the 30% buffer. Visitors retake photos, and a small share of renders are rejected by the check that protects the CSD crest and wording. Each of those costs a full render again, so renders always exceed posters handed out.
Monthly API spend
There is no subscription and no minimum. The API is billed purely on what is generated, so a month with no events costs nothing.
| Level of use | Posters / month | Flash 3.1 · 1K | Flash 3.1 · 2K | Pro 3 | Budget (Flash 1K + 30%) |
|---|---|---|---|---|---|
| Occasional — a few event days | 500 | HK$270 | HK$400 | HK$550 | HK$350 |
| Regular — roughly weekly | 2,000 | HK$1,080 | HK$1,600 | HK$2,200 | HK$1,400 |
| Busy — most weekdays | 5,000 | HK$2,690 | HK$4,020 | HK$5,500 | HK$3,500 |
| Continuous — daily booth | 10,000 | HK$5,380 | HK$8,030 | HK$11,000 | HK$7,000 |
| Peak campaign | 30,000 | HK$16,150 | HK$24,100 | HK$33,000 | HK$21,000 |
For scale, 10,000 posters a month is roughly 330 a day, every day — more than one staffed booth can physically serve at 22 seconds each, so those rows imply multiple booths. Realistic recurring use for a recruitment campaign sits in the first two rows, making HK$300 to HK$1,400 a month the range to plan around.
Two things that do not change with volume: there is no standing cost in a quiet month, and there are no volume discounts at these levels — cost is simply linear in posters produced.
Where the money goes
| Item | Per poster (US$) | Share of cost |
|---|---|---|
| Gemini image API (Flash 3.1 · 1K) | US$0.069 | ~97% |
| Cloud Run compute (~22 sec) | US$0.0013 | ~2% |
| Storage and download traffic | under US$0.0001 | under 1% |
Posters are deleted within 24 hours, so storage never accumulates, and the service scales to zero between events.
Which model to use
Flash 3.1 at 1K is the right default — both the cheapest option and the quickest.
| Flash 3.1 · 1K | Pro 3 | |
|---|---|---|
| Cost per poster | HK$0.54 | HK$1.10 |
| Wait per visitor | ~22 sec | ~43 sec |
| Posters per hour, one booth | ~160 | ~84 |
Throughput matters as much as price: at a busy stand the slower model nearly halves how many people can be served.
Pro 3 is the stronger model at rendering text, but that advantage does not apply here. The poster’s crest and wording are never taken from the model’s output — they are preserved from the original artwork — so the only thing Pro is being paid for is the figure itself.
Basis for these figures
Calculated from Google’s published Gemini API pricing, September 2026, which Google publishes and bills in US dollars:
| Model | Image output | Input |
|---|---|---|
gemini-3.1-flash-image |
US$0.067 per 1K image, US$0.101 per 2K | US$0.50 per million tokens |
gemini-3-pro-image |
US$0.134 per 1K or 2K image | US$2.00 per million tokens |
Input charges add roughly US$0.002 per poster on Flash and US$0.007 on Pro, across both API calls. Hong Kong dollar figures are converted at HK$7.8 to US$1, so they move with the exchange rate.
These are calculated figures, not invoiced amounts. Treat them as a planning estimate and confirm against an actual bill after the first event. Google may also revise its API pricing.