Busses are replaced regularly anyway. An average bus in Germany for example is 8 years old, so 6-7% of all busses are replaced every year. Just buy electric busses when replacing the old ones instead of throwing out perfectly new combustion engine models. That's also more environmentally friendly, as a large part of its lifecycle pollution happens during the construction of a vehicle.

The other question is obviously highly dependent on the city (size, density, geology etc.) and the type of transit you're building (underground vs overground, separate rails vs. tram on streets etc). As a current example Hamburg is building a new subway line that'll go through the entire city (25km, 24 Stations, almost completely underground) is estimated to cost 15 billion €. So, depending on how mucch your city needs it could be anywhere between 10 and 100bn for a subway net. However, the national accounting will benefit 1.28€ for every 1.00€ that's spent, due to savings in travel times, fuel, cost for accidents and road maintenance, freed up real estate in the city etc. according to the calculations.

1. Approximately $500k-1.1M per bus if you buy from New Flyer. https://www.masstransitmag.com/bus/vehicles/hybrid-hydrogen-electric-vehicles/press-release/21212587/niagara-frontier-transportation-authority-nfta-nfta-approves-electric-bus-purchase-from-new-flyer

2. Depends on where you live. Start from $100M per kilometre.

• If you are in North America, multiply by 3.
• If you have to go through dense city, multiply by 3.
• If you need to build bridges over a river, multiply by 3, or tunnel under a mountain, multiply by 5 for those sections.
• If your city is full of NIMBYs, multiply by the number of groups that would put forward a stupid reason with the aim to kill the project.

replacing an existing fleet of diesel transit buses with all EV literally pays for itself in lower maintenance and operating costs, even though the vehicles themselves cost as much as 50% more.

a couple of cost examples: the 15 mile extension (to the southwest burbs) of the LRT line that connects the downtowns of minneapolis and saint paul will end up with a final cost of around $200 million per mile. new underground subway lines/extensions in nyc have been built at costs over $1-2 billion per mile.

Two recent examples from Finland, just numbers, you decide if it's a little or a lot.

When phase 1 of the "Länsimetro" subway extension from Helsinki city center to adjascent area and to the nearby suburbs was completed in 2018, it had cost 1188M€ for a 13.5 km route in challenging terrain, and 8 stations. To the best I could find, phase 2 completed in 2023 with 7.5km more route and 5 more stations, and cost 1070M€. This includes the infrastructure, not the vehicles.

The Tampere tram system (started from scratch) opened in 2021. Phase 1 route length was 15 km and 23 pairs of stops, as well as 10 new bridges and such, and cost 274M€+82M€ for the vehicles. Phase 2 is 95% done and 92% paid for, and currently the 7km, 9 stop(pairs) project is estimated to come to a total of 100M€+39M€ for the vehicles.

https://www.lansimetro.fi/kustannukset/ https://yle.fi/a/3-8727469 https://en.wikipedia.org/wiki/L%C3%A4nsimetro https://www.tampereenratikka.fi/tampereen-ratikka/ratikan-kustannukset/

Regarding 1 - data is hard to find, apparently electric busses are 50-100% more expensive, but with lower operating costs. How much is uncertain. (requires digging into agency reports in languages I don't understand)

The good thing is that you don't have to throw good busses away in order to switch. Busses have a relatively short lifespan, about 15 years. They naturally come up for replacement anyway, and then you can buy electric ones instead.