Air sealing is easier & cheaper than you might think. Most newbie DIYers spend way to much time chasing the small leaks, and are ignorant of the bigger leaks that really matter. The pros know where to look, and can usually identify the bigger leaks even without blower door testing.
But blower door testing makes it pretty easy to find the first 90% of the leaks. As long as your house isn't full of asbestos it's legal to run blower-door tests to chase down those leaks, but it's the big leaks are the most important, and they are pretty similar in most houses. Leakage detected by a blower door is not all of equal importance- those at the foundation and at the attic come first, since they are what define the stack height of the "stack effect" drive, an infiltration drive that that runs 24/365.
On wood-framed buildings like CA bungalows the three biggest and most important leaks (in no particular order) tend to be:
* Foundation sill & band joist leakage (usually more than all window and door crackage leaks combined.)
* Flue & plumbing (and sometimes electrical) chases that run from basement to attic unimpeded.
* Holes in the upper floor ceiling/attic floor plane for electrical fixtures (recessed lighting being among the worst offenders), and AC/heating ducts.
Balloon framing sans-insulation is sometimes an issue for older bungalows, but probably not in a circa 1950 unit, since top-plates to all framing and fire-blocking between floors became required by building codes in most locations by then.
Fireplaces are also a common source of infiltration drive in cooler climates, not sure if most homes in Riverside CA would have that issue.
With a DIY approach to air-sealing significant progress on the big holes can be made with a $100-200 in materials and 10 hours of sweat-equity, half of which is figuring out where the big holes are, which isn't always obvious to the uninitiated, but easy to find with blower doors. In 1.5 story bungalows kneewall attic rooms into vented attic-cubby-crawls have all sorts of air-sealing issues that aren't always dead-obvious. Air sealing as a service is often provided by better insulation companies for $1000-2000 who use blower doors & infra-red cameras to chase it, and guarantee some minimum before/after percentage reduction. But the bulk of the reduction is in the first 5 big holes, which are pretty similar in most homes. When you're starting out with an untreated sieve of a house the first 50-60% is usually dead-easy and obvious, so if you're so inclined, a DIY first stab at it is almost always going to be worthwhile.
As measured by the pros with a blower door the leakage is usually expressed in cfm at a code & industry standardized pressure difference of 50 pascals or "cfm/50" for shorthand. They would normally measure it by both pressurizing and depressurizing the house. For code compliance issues that get converted to a total air exhanges per hour number or ACH/50 by calculating the enclosed volum of the house (including the basement, but not a vented attic.)
Under IRC-2009 rules new housing had to be under 7ACH/50, which is more like a stripe on the floor than a hurdle, for houses built with 4x8 plywood or OSB sheathing, and paying any attention at all to air sealing, and dead-easy to retrofit to in most homes. Under IRC 2012 that has been bumped up to 3 ACH/50, which is still pretty easy to hit without remediation on new construction, but can be a bit tougher level to retrofit to in some plank-sheathed balloon framed older housing. A 1000' house with a full (but unfinished, and not included in the square footage) basement and 9' ceilings for both would have a total enclosed volume of about 18,000 cubic feet. A leaky house that size will often initially test at ~3000 cfm/50, which means a complete air exchanges occurs every six minutes, or 10 air exchanges per hour. But those houses are usually pretty easy to bring under 2000 cfm/50, and sometimes easy to bring under 1500cfm/50, either of which is a HUGE improvement.
It only gets to be a mammoth undertaking if you're trying to meet a fairly tight specification like 2 ACH50 or less, which would likely require new windows and a lot of work, especially if it's a plank-sheathed house with clapboard or shingle siding, as is the case for many pre-1950 CA bungalows. If you're luck yours would have been built with plywood sheathing, but often in that time frame it was 1x10 ship-lap laid on the diagonal for wall-bracing, with a layer of #15 felt (tar-paper) between the sheathing and siding. Even the latter can be tightened up a lot by retrofitting blown cellulose in the wall cavities over whatever the original insulation there is (if any), which has other benefits as well, but at additional expense. If your house has no wall insulation at all (pretty common in 1950) it's probably leaking like a sieve even if sheathed in plywood, and retrofitting blown cellulose would be worth it on multiple grounds. But blowing in wall insulation is a bit beyond where DIYers would typically go.
CA being a highly regulated state that subsidizes energy efficiency, your house might qualify for significant air-sealing & insulation upgrade subsidies if it has no wall insulation. (I'm sure contractors that offer both air sealing and insulation would be all over it.)
