I don't have much experience of the cost of training NLP models (I work mostly in Vision). But I think if you can get a product out with just enough accuracy to get the heads turning in your favour, you could always scale up the model later down the road. Alternatively, you could have donate button on the extension's settings page (which many extensions do), if you do get some donations you could use it to update the model later on. It could be crowd-sourced and crowd-funded simultaneously.