Use the camera's audio as the scratch track. Syncing is not that difficult. Play back the original a few times for the actor. Record the new one onto a different track in the timeline. Keep doing it until you like the performance. Line it up by eye, then play back the line with both tracks. Shift the new one left and right until the echo goes away.
You're right, achieving a very rough sync like this is not difficult. Unfortunately, it's also not acceptable to today's audiences. It was more acceptable to audiences for amateur films a decade or two ago and was even acceptable to a section of the paying public 40 or 50 years ago.
Sure the performance *can* suffer, but that's where as a director I have to find creative ways to get the actor where I need them.
The performance virtually always suffers and that's with very experienced top professional actors, Hollywood directors and best commercial ADR teams! What chance is there with very inexperienced amateur actors, amateur directors, no commercial ADR team and amateur/consumer equipment and facilities?
People talk about ADR like it's rocket science, but I've done it for three features and countless shorts because I didn't have access to an experienced sound guy on set.
It's not rocket science at all. It's just very detailed and tricky work which requires the right tools, experience and a great deal of time if one is to do it without degrading the quality of the film too much.
I also knew that the majority of hollywood films are something like 80% ADR, so I figured why not go all the way.
There are three problems with this statement: 1. 80% ADR would be rare, the majority are more like 40%. There are probably as many or more films with 20% ADR or lower than there are with 80% and 2. No Hollywood directors or actors like ADR or would ever chose to do ADR, it's almost always a case of the lesser of two evils and/or a last resort. A few Hollywood directors go to extreme lengths to avoid ADR. 3. What you are suggesting is not really ADR at all! It's essentially just dubbing with a guide track rather than actual ADR as employed by Hollywood. Hollywood uses specialised ADR teams with specialised pattern matching software which employ complex algorithms to subtly time-strech or compress individual phonemes or syllables in the ADR to precisely match the timing of the original production sound. This is where the "Automatic" comes from in "Automatic Dialogue Replacement"! In practice this software is not entirely accurate or without artefacts and can only handle a few words at at time, so it needs to go hand in hand with clever editing and various other manual tricks to achieve near perfect sync and requires a lot of time, knowledge and experience. Then of course is the issue of mixing this edited ADR to match with any existing production sound or to create the aural perspective to match the visuals, which again takes a lot of time, experience and resources if it is to be done convincingly. In other words, even if we were to accept your 80% figure, that would be 80% ADR, not 80% dubbed with a guide track!
The OP asked for a cheap mic, and the Yeti with this workflow is my suggestion. I realize it's not the "right" way, but it can produce desirable results.
The reason it's not the "right" way to do it is because it does NOT generally produce desirable results! Why do you think a "right" way exists? Of course this brings up the question of how we define "desirable", which is directly related to what audiences find acceptable. In North America, Britain and many other countries, what you are suggesting would not generally be acceptable and therefore would not be desirable, either by commercial standards or even by most amateur/aspiring pro standards. The cheapest, easiest and quickest way, particularly at the amateur level/aspiring pro level, of achieving what most would consider to be "desirable" results, is to do one's utmost to capture decent production sound in the first place, avoid ADR as far as possible and avoid dubbing at almost any cost!
G