The Visa cost stays at 50 USD. Nationals of just 40 countries are eligible for the visa, these are mostly western countries. Nationals of most neighboring countries, on the other hand, can visit these countries visa-free.
Univisa will be valid for 30 days and allows multiple crossings between the two countries (limited by passport pages as it gets stamped at every crossing!). If re-entering from a third country, a new visa has to be obtained, even if the previous visa is still valid. The only exception is a day trip to Botswana (most likely a safari to Chobe National Park).
The visa can be obtained on arrival at these airports: Lusaka, Livingstone, Victoria Falls and Harare. It is also issued at the two Kazungula border posts bordering Botswana with Zambia and Zimbabwe, respectively. Unfortunately, those arriving by land from other countries, like South Africa, Namibia, Mozambique, Malawi or Tanzania still cannot obtain the visa.
Update January 2017: Univisa issued again
Last year we wrote about KAZA UniVisa, the tourist visa that allows tourists to visit both Zambia and Zimbabwe, mainly targeting those visiting Victoria Falls. In bad news, the issuing of UniVisa has been suspended, at least temporarily. There has been no official statement by either country, the issuing of UniVisa was stopped after UniVisa stickers ran out, first in Zambia, then in Zimbabwe. No information is available whether the visa will be reinstated.
UPDATE JANUARY 2016: UniVisa no longer issued
UPDATE JANUARY 2017: Univisa issued again
A tourist on African tour will have to obtain visas to many countries visited, which will be a costly and time-consuming affair. To somewhat alleviate this problem, new cross-border visas have emerged. Recently I wrote a blog post on the East African Tourist Visa (covering Kenya Uganda and Rwanda), There is also UniVisa between Zimbabwe and Zambia (also known as Kaza Visa), which was launched in late 2014. UniVisa will be valid for 30 days and allows multiple entries between these two countries. Only certain nationals are eligible for Univisa (listed here). UniVisa is especially handy for visitors going to Victoria Falls, as travellers can now easily cross the border without having to pay for two separate visas. However, the visa is still not available from most border posts and some international airports.