Question, how much would it cost (approximately) to have a foundation poured in San Diego (for a 3000 sq ft house)?
20k minimum. That is for a flat building area with no soil issues and no garage slab. Realistically it is likely to be higher and some foundation types need a skirt and code built stairs and landing at each entry because modular homes are often elevated.I would suggest you plan on nothing less than 30k. The Modular home builder should provide you with a few generic foundation plans for California. You must figure out which one fits your site and home.
Check out the cost to have the right to hook up to water and sewer it ranges form 5k to 15k for most areas last time I checked(that doesn’t include materials and labor). School fees are a few bucks per square foot. Some areas there is a 5k transportation impact fee. All this adds up. I think your biggest issue is going to be finding a lot in Bonita for the price range you are talking about.$150k is almost impossible anywhere in the county and even a $200k lot is going to have some issues that are going to affect building costs and/or will come with other negatives. I doubt you will find a reasonably build-able lot anywhere except semi-rural & rural areas for near that.
This is not to knock the wind out of your sails, these goals are always a process and will usually take several turns.